The Citadel will make a comeback for Mass Effect 3.
Speaking recently to OXM, lead writer Mac Walters explained that cut content from ME2 (evolving certain story arcs) will make it into the third game. If you looked at the recent plot-leak online (shame on you) it should be obvious what this section is.
After that plot leak BioWare went on record to say that they're analysing feedback from the plot-leak to amend certain aspects of the final game. Don't expect sweeping changes - the game is only three months away -but some significant tweaks will be made.
Perhaps the biggest series shift for Mass Effect 3 is multiplayer, which appears as part of the game's Galaxy At War theme, as a glorified horde mode. BioWare have gone on record to say they looked at putting it in the original game, but that wasn't possible.
Interestingly, your multiplayer character gains levels as you play ME3, and you can send them to fight the Reapers in the main singleplayer game. This is optional, but it cleverly links both parts of the game.
Mass Effect's combat can't compete with Gears of War, so as a pure shooter it's far from perfect. Shepard's slow turning circle makes him clunky compared to Marcus Fenix and co, and the enemy AI isn't as sharp or dynamic.
Mass Effect is, after all, a role-player as much as a shooter, and as this mode focuses entirely on the action side of things, it loses out.