First and foremost, I've noticed that MvC3 is a lot slower than the previous game. The fights seem to creep along at a more mundane pace this time around. It just doesn't feel like Marvel vs Capcom.
At least some of the returning characters provide a much needed connection for veteran players: Captain America, Tron Bonne, Magneto, Dr. Doom and some other classic choices are largely unchanged. They've been tweaked a bit, for sure, but if your favorite fighter from MvC2 is in the new game, you'll at least be able to jump in with nary a stumble.
Some of the new fighters, on the other hand, aren't working quite so well. Resident Evil's Wesker felt notably overpowered in the preview build I played and should probably be balanced in the final game. He could instantly punish any opponent, no matter where the character was on the screen. He could even punish characters that were off-screen, too. In one match, when Storm missed her Super as she stood two screen lengths above me, I easily found her with Wesker's Super, a generous application of his custom shiatsu massage of punches and kicks.
Another broken newcomer is Arthur. His limitless well of projectiles (seen in the video above) ensures that most characters can't get near him. He can throw daggers, javelins, holy water, axes and just about anything else you've seen in the Ghosts 'n Goblins games. He's easily the best keep-away character in the preview build, but it's incredibly frustrating that there's no check or balance for his tactics. Let's add that to the list of tweaks needed in the final game, Capcom.
The best thing about Marvel vs Capcom 2 is that as chaotic as it is, it works -- it never feels blatantly unfair, or that you've lost a match before it's even started because of the opposing lineup. Sure, some of the fighters are just plain amazing, but none are invincible -- good strategies rein supreme. In my preview of Marvel vs Capcom 3, however, it seemed like each match was a toss-up, a coin flipping in the air to determine who's going to win the match. It's just too chaotic: Sometimes I mash buttons and win; other times I try to play it technical and get crushed. I can't strike a consistent balance, and that's frustrating for someone who has been playing fighting games since the genre came to be.
Though often frustrating to play, the flaws I perceived in this version of Marvel vs Capcom 3 seemed evident and would appear quite fixable. It looks fantastic and can even be fun at times, but for dedicated fighting game players, Marvel vs Capcom 3 seems to pose a difficult proposition as is: Ignore the technical knowledge and abilities you've accrued playing various fighters over the years and agree to play this one by its own very specific set of rules -- or go back to playing your old favorites. If Capcom fails to address certain issues with Marvel vs Capcom 3 before its mid-February release, I suspect many will opt for the latter.