A less stripped down set of gameplay mechanics.
Seriously, I had a blast with ME2 for about 20 hrs. Which, granted, is really good if one considers other new releases. The problem is that I couldn't stomach another playthrough. Why? Same reason I couldn't for the first one either: The classes don't feel distinct enough. Play the game with one class you've pretty much played the game as all of them. I want to go through again, making different decisions, but the combat missions get so monotonous so quickly, I just can't do it.
I think that this is less a problem with the classes and more a problem with the level design which essentially forces you to use the same tactics over and over again. If I have one fault with BioWare, this is it. They are awesome at giving the player choices on how the story plays out, but really fall short in giving you options in combat. KOTOR was probably the best they've done at making the different classes play differently. That's the only BioWare game I've been able to put more than a hundred hours into.
Compare that to Oblivion and Morrowind, in which I've put 100+ hours into single characters.
What I want in a sentence: Make the action as dynamic as the story.