So far Steam description of 2.0 is the most comprehensive
Completely New, Extensive Tutorial System - The new tutorial helps players fully enjoy the game from the very beginning and systematically explains the depth of The Witcher 2’s combat and gameplay mechanics. It allows gamers to be gradually and smoothly immersed in the game world and Geralt’s adventures.
Special Dark Difficulty Setting - A new, intensely challenging difficulty setting that provides the ultimate challenge for players. On the Dark setting, The Witcher 2 is nearly as difficult as Insane, but allowing players to save the game. It also provides a completely new set of dark-themed gear with very high stats such as:
18 new equip-able items including
3 new sets of dark armor (light, medium, and heavy)
3 new sets of dark swords (3 silver, 3 steel)
Other Dark themes/elements in the game
Completely New Arena Mode - Infinitely re-playable and progressively more challenging, Arena lets you hone your combat style and techniques against all the different monster types from the game. Dispatch wave upon wave of progressively more difficult enemies, earning awards that you can use to upgrade abilities, weapons and armor, and to test alchemy. Hirelings can also assist you in battle for a price. Combat waves will continue infinitely until your character is defeated, when you’ll see a statistical summary of results. Players can share their results with their friends by uploading this summary to the forums or to Facebook. Arena Mode includes:
New location, with a dynamic, responsive audience reacting to the events
A new monster type, Werewolf
Waves of different opponents, looped with increasing levels of difficulty
3 different prizes to choose from in each wave
3 new characters (mage, knight, and dwarven fighter) that can serve as allies during clashes
A shopkeeper offering gear and enhancements
3 new GUI panels designed specifically for the mode
New dialogue
New background music
New Witcher Forum and Facebook integration
Technical Bliss - Version 2.0 includes all previous and new technical and gameplay enhancements such as:
40+ new technical and gameplay enhancements
100+ previous technical and gameplay fixes
All 9 previous DLC packs - Troll Trouble, Roche Commando Jacket, Ultimate Alchemy Suit, Ultimate Magical Suit, Ultimate Swordsman Suit, Finisher Pack, Mysterious Merchant, Barbers and Coiffeuses, A Sackfull of Fluff
The Witcher 2, in its most definitive version 2.0, will be available on the 29th of the September through automatic update for free, for all existing and new users (all new installations will update to version 2.0 automatically during game installation process).