My list of things I want changed that no one will ever read...
1. Bring back the RPG elements
2. Um, that's pretty much it
ME2 was just so damn linear. Guns, gear, classes, characters - all the upgrades were predetermined, so I felt no sense of self-determination. All of the areas were narrow, point A to point B affairs, so I felt no sense of exploration. Even the story had only a few instances of deviation, so I felt no sense of authorship.
Don't misunderstand; great game, great action, great narrative. I'll be amped to play ME3 even if they change nothing. But I think they overreacted to some of the criticisms of ME1. When someone says your inventory system is sloppy, you should focus on making it suck less - as opposed to, you know, completely jettisoning it from the game.
Oh, one thing most people should agree on: the concept of resource gathering is just inherently flawed. ME2 was plenty long and satisfying without the 2-3 hours I spent mindlessly grinding planets for space gold. If you're looking to evolve the genre, feel free to get rid of the parts no one likes before you destroy the details many enjoy. Weave stat progression into the story missions and side quests players want to see anyways.