This mod is a more strategic version of the regular game, but with many new bosses, skills, and challenges. I would highly recommend forming team of 2 or 3 as the bosses can be very tricky alone.

Some general info:

The first thing you willnotice is there are no Health or Mana potions here. Healing is instead done via an innate skill every class has. This becomes less effective as you level up. However, spending points in energy, dexterity or strength will increase it's healing rate.

All resists are capped at 50 but can be improved with equipment.

Next is the lost soul prison.
This is a handy little tool used for number of things. Firstly, it records kills directly by you. Spells are included but minion kills won't count. There are cube recipes later in the game that require you to have trapped a certain number of souls.
Along with counting slain souls. It also turns raw materials into components for crafting, it can also be upgraded and imbued with new properties. See the documentation for more details.

Items:
Items are alot more unique now. (Due to the fact there are no (or very few) "Unique" items)
Rares have been improved, along with tempering to create your own custom item to your specs. All original runewords have been removed and replaced to a lesser extent.
See the documentation for more details

Skills:
A number of new skills for each class and alterations to other skills to create new build options
There are no pre requirements for skills and they are improved with stat synergies mostly.

Enemys:
many new monsters added, and many new bosses. Enemys hit hard and can take a beating in return. You will need to use your brains and know when to fall back to recover.
Bosses come with a number of tricks uptheir sleeves. First, if they are unhurt for 20 seconds.They recover to full health. This is so you have to killthem in 1 go. No more half dead bosses after you wipe out!
Second, they have a number of counter attacks. some weak, some not so wak.Some may summon support and somemay curse you.
They also have another special counter that renders them curse immune, and grants 100% attack speed, move speed and damage. This lasts 5 seconds at a time so you may want to back off when this happens.

Changelog:

0.21f Changes:
Rune dusts added,
barbs concentrate damage reduced.
Druids maul damage and fury damage increased.
druid summons numbers reduced.
necro summons cooldown increased.
Block chance on shields reduced to increase the need for dex.
all 2 hand weapon damages reduced by 20%
reduced fps drop on immo and meteor stacked fire.
made act end boss fights slightly harder and longer (except normal andariel)
Increased mana cost on most spells that require heavy energy investment so the blue orb is still important.
added rune dusts (new socketables). These are used for runewords but the new runewords are still to be completed. I'll add these now so people have a chance to find some and stock up.
trash damage in normal reduced by 15%, nm and hell damage remains the same.
diablos nasty death counter removed
ligtning bolt mana cost reduced
bone spear number reduced
readded cooldowns to summons
reduced barbs overall damage by 10%
old bnet runewords have been removed 0.21e changes:Arctic storm buffed by 50%
blast wave next hit delay fixed
act 2 def mercs now have salvation
revive nerfed abit
ww now does 2/3 weapon damage along with a 50% damage penalty at level 1
frozen orbs damage dropped 5%
lightning bolts dmage increased
ancient Kaa is now a boss and will be required soon for an optional sidequest
new boss guards the portal to arcane sanctuary
Grizzly's Flare Strike skill changed to monster version with higher fire damage and lower phys damage
treewood nerfed slightly