A COUPLE SHOUTOUTS!
Everyone at Phrozenkeep (and the more recent discord channel). For 20 years PK has maintained all the guides, tutorials, and resources that I could have ever wanted.
The Diablo Mods discord. Honestly if I hadn't somehow stumbled across it, I probably wouldn't have wanted to get back into modding again.
Personal shoutout to andsimo, perhaps the greatest D2 mod historian on the planet. Thank you for all the work you've done curating a space for us to enjoy all the work modders have done over the past twenty years.
A whole bunch of other folks who played a role in this mod renaissance, whether they know it or not- jlynnes27, imsully2, HarvestWombs, Necrolis, Bgrmystr2, Eggsy, Haste4653, ultimategrayfox, SenpaiSomething, Tenshi no Gado, Birds and Ram, Primeval, and a bunch of other people who have been kind, gracious, or thoughtful.
The OG Blizzard team, for making a game that I just can't stop thinking about even 20 years later.

CONTENT ADDED THANKS TO OTHERS
Increased inventory/cube size thanks to Mhoram/Ravenswolf
Added full merc inventory by SVR
PlugY by Yohann, praise be
Added D2GL for high resolution support (default: 1600x900) thanks to rnd2k, bayaraa, and everyone else: Mir Drualga, Bolrog, Libretro, Brian Paul, G-Truc Creation, Timothy Lottes, Omar Cornut, Sean Barrett, Austin Appleby, Chlumsky, laurentlb
The hi-res system also comes equipped with HD fonts and have mercy they look fantastic. Also it fixed "5". We truly are living in the future.
Extended level system courtesy of: Afj666, converted to the D2Mod System by SVR 2004, expanded by Myhrginoc 2007, 2021
Additional cube recipe support by Havvoric, VillageIdiot and kingpin
Support for new .tbl files thanks to SVR and Allan Jensen
SVR is also responsible for D2Mod itself and its Utility plugin
Massive QoL thanks to D2Tweaks. Huge shoutout to Revan, recently updated by rnd2k. Enables drop notifications, autopickup, Ctrl+click to move items, floating damage numbers, and more!
Cleaner skill trees largely thanks to RicFaith
More readable gems thanks to Myhrginoc's GemFix plugin
Included PoE-style "show items" (button press -> toggle)- D2Alt is courtesy of rnd2k
A number of significant qol changes thanks to SEO-master and rnd2k's SEO.dll d2mod plugin. Great stuff!
Added new item graphics thanks to HarvestWombs, StarGazer79, AlphA, riparious, Joel, taarna, Ogodei, intravenous, cla$$ics, and Shadowflare. Many of these graphics are from other games- mainly Grim Dawn and Path of Exile. If you haven't played them, go do so! They are both great for very different reasons.
A bunch of new item stats thanks to Talonrage (why reinvent the wheel?)
"Borrowed" some inventory graphics from online resources. Don't worry, I'll give them back as soon as I'm done with them.

OPTIONS AVAILABLE TO PLAYERS
The mod comes bundled with a bunch of qol additions, and many of them have .ini files that can be modified to fit the player's preferences.
Currently, these include:
d2alt.ini
d2gl.ini
d2tweaks.ini
seo.ini (with caveat- don't mess with the d2alt setting here!)
There are other .ini files included with the mod but I strongly discourage players from modifying these, as it can lead to crashes or a poorly balanced gameplay experience.
If you have questions about these .ini's, please ask in the discord or on the subreddit.
Ultimately, the mod is designed to not need modifications to any of these, but I recognize many players want to adjust the game a bit to their liking, so I have left these .ini's available. Undesired gameplay changes are the responsibility of the user.


MY CONTENT
Removed the durability system- all items are indestructible. The consequence to this is that no items will spawn ethereal. Oh no. Anyways.
Also removed ID scrolls (in most places- some will still exist to keep the game from crashing). D2Tweaks will auto-id every item you pick up. System is antiquated and I hate it. Less time clicking scrolls = more time clicking loot.
Removed most traditional cube recipes. You can no longer upgrade gems or runes. 
All quantity-based items (arrows/bolts/throwing weapons) are infinite.
Ammo (arrows/bolts) can now spawn magic and rare. 
Throwing potions can now also spawn magic and rare.
Armor defense is normalized- it no longer has a range it rolls between.
Armor no longer has a hidden penalty to movespeed. I wonder how many players knew it existed in the first place.
Some armors require dexterity.
Shields have significantly lower chance to block. For a sense: Bucklers have 1% block. Elite Paladin shields have 10% block.
Level requirements normalized: normal items have no (level) requirements, exceptional items require level 25, and elite items require level 50. The only exception is the Diadem, which requires level 75. It's special. Note these requirements only apply to base items; magic, rare, and unique items may have different level requirements.
Superior items are buffed and normalized (all superior armors get %defense, all weapons get %damage). Added a couple stats superior items can spawn with for more variation.
Cube recipes, unless specifically stated on crafting items, are always based on 100% itemlevel (assuming it is crafting a base item).
Thorns (and all derivatives) now trigger when you are attacked and not when you are damaged, making it much easier to trigger. ctc (when struck) are unchanged. Additionally, there is now thorns of all damage types (minus poison).
Axes deal bonus damage to demons (as in the original Diablo, did you know?).
All curses, and shrines, are now stackable. Mostly qol for shrines, but could spell some danger in curse-heavy areas.
Normalized character starting stats, hp/mana per level, and hp/mana per relevant stat point. All characters start with 25 in each stat, 55 HP/25 Mana, have 9/10 walk/run speed, gain 3/2/3 life/stamina/mana per level, gain 2 life per vitality, and 2 mana per energy.
Mana regen greatly increased; by default a character's mana regenerates from 0 to full in 10 seconds. Mana costs significantly increased to accommodate. The goal is to promote constant mana expenditure without making it frustrating to run low on mana.
Effectively removed stamina drain. Technically stamina drain is set to 1 (vanilla is 10), but it's unlikely you'll actually run out of stamina by running. Not that you really want to anyways.
All (most) melee weapons deal splash damage in a small area around the target. Caster weapons (wands, knives, scepters), ranged weapons, and throwing weapons do not inherit melee splash. Instead, caster weapons offer an array of bonuses to, you know, casters. For now, staves are considered hybrid weapons and still grant melee splash.
Melee splash has a very small radius (1 yard) and gains +1 radius (1/3 yard) per 100 (strength+dexterity).
Your current melee splash radius can be seen on the description of your default attack.
Players gain 3 stat points and 2 skill points per level.
Adjusted (read: increased) monster stats across the board. Generally speaking, individual monsters have doubled hp and ~50% more attack rating and defense. Their damage is slightly increased.
Added info scrolls to the game in an attempt to explain the sweeping changes to some game systems. They are not exhaustive but cover what I think are the biggest hurdles for new players to grasp.
Please feel free to suggest new info scrolls on the discord or subreddit.

Created the Mastery System for skills. Instead of attribute points on leveling up, putting skill points in "Mastery Skills" will allow for players to increase their stats and help differentiate each class.

Each character has Mastery skills which grant "Mastery" (usually alongside some other bonus). The character's usable skills require a certain total Mastery level in order to learn them. Total Mastery level is shown on the 3rd stats page, thanks to PlugY. There are 10 masteries in total, each going up to level 10. This gives a theoretical maximum Mastery Level of 110. Mastery only scales with hard points in Mastery skills.
Your normal level 1 skills only require 1 Mastery, but the highest level skills a character can invest in require 50 Mastery.
Mastery progression for skill "tiers" is as follows: 1->3->10->15->30->50

Overhauled character skills. Instead of 30 skills, characters now have access to 22 (2 trees with 11 skills each). These are mostly reworked vanilla skills, though some are so reworked that they will feel brand new.
Skills typically require multiple skill points to level up, to account for the player gaining multiple skill points per level.

Many spells now deal a portion of weapon damage in order to allow casters to benefit from procs, leeching, knockback, and other effects. Note that due to this huge buff, many of the sources of those stats have been greatly diminished. In order to not penalize melee characters, many new stats now specifically focus on melee.
Please note that this change means that the character screen will not be accurate for almost every spell in the game- it does not correctly account for spells dealing weapon damage components. Please read the tooltips in-game for skill damage; I have painstakingly calculated spell damage manually so it is displayed correctly.

Significantly expanded the usefulness of skill descriptions. All skills now display much more information, making it easier to determine exactly how skills function.
Many skill descriptions have color-coded aspects, as follows:
-blue text typically displays required weapon types or skill states in order to use the skill, and any speed scaling the skill has (attack/cast speed)
-purple text shows what "type" of skill it is, out of a vast range: attack, spell, ranged, buff, summon, curse, channeling, etc. in an attempt to make it clear what a skill does without having to invest in it
-gold text is "flavor text", sometimes leaning heavily into lore, sometimes more just an outright description of the skill.


Synergies no longer exist. Skills scale individually (with hard points), or with some of the new stats. Return to 1.09. Return to monke.

Heavily modified socketables. Added three new gem types. Reconstructed runes from the ground up- there are now 20 runes total. They start dropping at level 40.
A new, high-level gem has been added for each class. Currently these basically do nothing and are placeholders; they will eventually be used in endgame crafting.
Due to rune changes, Countess no longer drops runes (as they would be vastly above her level in both Normal and NM). Instead, she now drops GUARANTEED crafting mats.

Runewords all heavily modified and custom-built from scratch. Currently, all are disabled as runes should not drop in any "finished" content. Once normal difficulty is finished, runewords will be enabled slowly through the alpha.

Updated uniques. Pretty much every vanilla unique now has an actually unique modifier on it. Most +skills or charges on uniques have been changed to oskills. Plenty of items with auras, scaling stats, or procs. etcetcetc. I won't spoil anything here, though! Enjoy finding them :)

Added new stats, cleaned up display of existing ones, and enabled variations on current ones. These include, but are not limited to, Minion Efficiency (each minion scales differently), random oskills, attribute%, random reanimates, [stat] based on Mastery Level, and more. There are approximately 100 new stats to be found on skills and items.

Rebuilt magic/rare affixes from the ground up. For the most part this leads to much more interesting and balanced items, though currently the pool is small so many items will feel "samey".
Magic items have the highest level affixes available (comparison: magic weapons can roll 300-400% Enhanced Damage, rares can only roll 201-240% Enhanced Damage)- this means magic items are sometimes what you'll want to chase in endgame.
Rares pull from a diverse (but much smaller than vanilla) affix list, but affixes are phased out of the pool much faster than vanilla (at item level 67+, there are only 3 possible Enhanced Damage affixes that can roll on a weapon)
Affix pools are constructed in such a way that items will often be "good", sometimes be "weird", and rarely be "great"- with the possibility of getting "stacked" affixes (i.e. a weapon with %damage, %damage+%ar, and +damage), well-rolled or well-crafted rares are best in slot for most builds.
Affixes are designed to never be "useless".

Added new crafting materials.
Chaos Orb: rerolls a rare item (thanks, PoE!)
Vaal Orb: corrupts an item, modifying it unpredictably but making it unable to be further crafted through most means (thanks again, PoE!)
Orb of Alteration: rerolls a magic item (okay I'm done thanking PoE for crafting material ideas)
Augmentation Dust: upgrades a normal item to a magic item
Eye of the Divine: rerolls a unique item (will not reroll into a different unique, just rerolls its stats- meant for chasing perfect rolls)
Bloom of Eden: upgrades a normal/magic item to rare
Vial of Dragonblood: upgrades a magic item to crafted; allows for moderate control over additional affixes using gems (add any 2 perfect/class gems) ***CURRENTLY NOT IMPLEMENTED***
Goddess's Crystal: upgrades the base type of an item (i.e. Cap to War Hat), any rarity- item will retain all its mods

Created a new avenue for crafting: Seals of Synthesis. These items are very powerful, but have strict requirements. They will convert a given affix to another given affix (or similar behavior), but the initial affix must match the requirements on the seal! Fulfilling the requirements can do one of two things: upgrade the affix, or change it entirely. While it's usually pure upside, there are some sidegrade cases. These items are usable on all item types, allowing players to get very creative with their rare, set, or unique items. I am sure I have not thought of all potential use cases, so explore and exploit!

Created storage containers for new and existing crafting materials- one for magical essence, one for gems (stores quality only), one for runes, one for various other materials (mostly new ones). Items can be cubed to store them, and the container can be cubed with an extractor to remove the material. In the case of essence, the container can be used in place of different quality items for crafting. Don't store items you may want to keep as essence, as they are not retrievable.

Added Monster Infrequents.
Every monster in the game has a special item it can drop, usually a weapon or armor. These items have some minor built-in modifiers.
The system is planned to be expanded to every individual monster in the game. Yes, that's something like 500 items.
Superunique monsters drop, well, superunique monster infrequents. These are effectively unique items, but with wildly unique stats. Almost every Monster Infrequent that drops off a Superunique has a modifier that won't spawn anywhere else.

Added influenced items.
Influenced items drop solely from Act Bosses and can roll modifiers only available on influenced bases. Each boss has a wide pool of influenced items available, including armors, rings, and weapons.
Influenced items can roll "regular" mods, but also have a chance of rolling special influenced modifiers. These are far more powerful than standard mods and are generally themed around the boss that dropped the item.
The Influenced system has planned expansions as end-game content is rolled out.