First and foremost is the game's new skill set customization which is in turn tied to a new armor system. In a nutshell, your armor choice will affect your usable skills by rendering certain abilities ineffective and leading to highly specialized character roles (as opposed to the current game where every character can learn every skill, given enough grinding).
Skill and attribute gains have also been tweaked, and players who focus on skilling up a single role will excel at it in short order through "casual play." The catch, though, is the inability to have more than one specialization active at a time. Crafting has also been overhauled, and the world map has been "completely changed." There's more big news where that came from too, including UI tweaks, alignment tweaks, incapacitation, dungeon and magic school redesigns, and new combat mechanics like momentum and crosshair wobble. Read all about it at the official Darkfall website.