You didn't think gameplay would be exactly the same, did you? MvC3 has air tags now for easier tag-ins and a better team experience overall. There's also the new X-Factor system, which promises to change MvC strategy as you think you know it.
Essentially, X-Factor is a mega-steroid: it gives you a power and speed boost, and even regenerates health. Any character can use it at any time, simply by pressing all four attack buttons at once—it'll even cancel whatever combo you're in a the time. But here's the catch: your team can only use it once per match. Not only that, but the X-Factor's benefits get better the worse off your team is. If you have all three team members active, the boost will only last seven or eight seconds; if one of your players is down, those boosts are doubled. And if you wait until you have only one player left, you enjoy a boost of around 20 seconds that's three times more powerful than the weakest X-Factor. The question, of course: when will you use it? If you burn it too soon, you won't have it when you need it—but if you desperately need it to finish off an opponent, you may not even get the chance to activate it before they combo you into submission.