From an RPG character standpoint, we've planned more character-building choices (the player-character can be optionally male or female), we've unified some of the game systems and added some interesting player-character capabilities. We feel like we now have a much clearer idea about which aspects of DX1 were important to players, what worked well and what didn't.
The game environment itself in DX2 is something that has us completely excited. Our plans include a higher-fidelity world, with weather, greater object density, more capable (and clearer) unit behaviors and a deeper physics/object-interaction simulation.