Fast paced to a fault.
I've spent a lot of time with fighting games, and although I've never been above average at them, I rarely have trouble learning the ropes. Marvel Vs Capcom 3 kicked my ass, and it still does from time to time. While there is certainly tons of fun fast paced fighting to be had, actually getting any of that fun is a challenge. Sure, there's a training mode to practice in, and mission mode supposedly teaches you some of the finer points of each character, but unless you have a basic idea of how the fights work when you step in, you're going to be lost. Mission mode just displays the name of a move, and in order to see the input you have to pause the game and look at the "mission objectives". There is no way to display the input on screen during the actual gameplay. Further dragging down the potential for new players to actually learn anything here, after the second mission you are suddenly given combos that require knowledge of how cancels and exchanges work, and some of them are almost 20 moves long. The game basically tells new players to screw off.
Admittedly, Capcom tried to make the instruction manual fairly comprehensive, and to an extent, it does help ease you in. But in order to really stand a chance, you're going to have to either have that previous knowledge, or look online.
Once you've figured out how the basics work, and the flow of combat, things get much better. The game moves at a breakneck speed, and a variety of techniques allow the tide of battle to quickly turn. Hyper combos can be performed once you have at least one stock of your super meter, and 3 stocks allow you to use all 3 of your characters hyper combos at once. You can also spend 2 stock to input one hyper combo during another, canceling into it for even more damage. Pressing all of the attack buttons at once activates your X-Factor, which you can use once per match. X-Factor makes you faster, stronger, and recover health quicker. Depending on how many characters on your team are still conscious, the duration and impact of the benefits changes. Using it with your last guy gives huge benefits, while using it early on gives little, but that little might just give you the lead you want to take as quickly as you can. The bumpers call in your partners for their assist move, which is chosen at the character select screen, and holding their corresponding bumper swaps your current fighter out for them. X is a light attack, Y is a medium, and B is a heavy. The A button is called the "Exchange" button, and for the most part it acts as a launcher that you can use to knock your foe into the air. Once in the air, you can quickly jump up to follow up with more hits, and then use the exchange button to tag out for a partner who will tag in with a kick,allowing you to keep the air combo going. The point is, the game is complex as hell, and the learning curve is steep.
My only complaint aside from how high the barrier to entry is would be the online play. Capcom took a step back from Super Street Fighter 4 here and I don't have any idea why. Player matches can have up to 8 people in them, but the 6 people not playing at that moment are stuck looking at a static screen rather than spectating the fight. Ranked matches rarely connect, so ultimately the best way to get into a game online is to start up arcade mode with fight requests on, so as soon as a match is made you are paired up without having to deal with disconnect errors and menus.
If you're willing to deal with the huge learning curve and strange design choices, Marvel VS Capcom 3 can be one of the most entertaining fighting games on the market. Just be willing to get your ass kicked for quite a while.