Right at the beginning you learn that you control a generic mutant and will develop him/her throughout the campaign. The disappointment as a comic fan came from not being able to play as the likes of Wolverine, Cyclops, Gambit, Juggernaut, Magneto, etc. or any other mutant. There are so many mutant encounters, but they only act as AI the entire time. The plot is a simple X-Men vs Brotherhood battle, and you can choose at some parts to join either faction, causing different game paths.
You do however get to at least acquire all kinds of mutant powers from those you encounter in the campaign. You can collect power-up suits with special powers or unlock powers w/ story progression. It’s fun experimenting with different offensive and defensive powers. Sometimes you’ll want to equip Wolverine's attack damage or maybe equip Collossus’ metal suit for defense, for ex.
You can also choose from different mutant powers yourself as you progress. There is lots of depth in upgrading and leveling up these powers. The variety was there, but they weren't all that cool to use. Also, depending on which path you choose at points, you can get new abilities from either X-Men or Brotherhood mutants.
The lack of online play and co-op was a disappointment for sure, and the campaign was very short. The levels are too straight forward, and you'll encounter the typical boss battles you'd expect here and there. But you can seriously button mash X the entire game to beat up every enemy. Sidequests were all the same--kill X amount of enemies...like you couldn't come up with anything more bobby-generic than that? The lack of attack combos didn't help either, but it was just wasn't challenging enough. There is very little replay value for me, unless you want to start a new campaign to see new powers or get achievement points. You can get like 650G just from beating the game, so it's good for the hunters.