My only experience with MvC3 so far is a bit of single-player, but it seems pretty great. My observations from maybe twenty minutes of play:
-Morrigan is an automatic main (as she has been in
DarkStalkers,
Night Warriors,
Vampire Savior,
Marvel vs. Capcom,
Marvel vs. Capcom 2,
Super Gem Fighter: Mini Mix,
Super Puzzle Fighter II Turbo,
Capcom vs. SNK 2, and
Tatsunoko vs. Capcom), but the new aerial-forward and ground-to-air Soul Fists make her even better. I didn't get a chance to use Astral Vision, but I hear it's insanely good as well.
-My dad was laughing his head off at Deadpool. "BANGBANGBANGBANGBANGBANGBANGBANGBANG!"
-I didn't have enough time to see how retardedly good M.O.D.O.K. is myself, but he still seemed like he'd be fun.
-Hsien-Ko's way fun; as Jim said before, they kept pretty much her entire moveset, so she was easy to pick up and play. Guillotine Swing <3
-I didn't get to try Arthur, but if he's as much of a
Touhou character as he's been made out to be, he'd probably be a perma-main as well.
-Surprisingly, I didn't have any problems with inputs on the Xbox 360 D-pad. Maybe it's because I've been using the Dreamcast D-pad (which, and you guys aren't going to believe this, makes even the 360 D-pad look good) and the GameCube D-pad (which is in no way suitable for fighting games) lately.