Edge asked in the comments for a little more information about how Marvel Ultimate Alliance plays. Apparently "button mashy" wasn't a very insightful analysis, go figure ;)
The engine used for MUA is called the infinity engine, which was created for Baldur's Gate Dark Alliance and adapted for: Baldur's Gate Dark Alliance II, Champions of Norrath (and its sequel return to arms), Fallout Brotherhood of Steel, X-Men Legends (and its sequel Rise of Apocalypse) and now finally Marvel Ultimate Alliance.
Basically the infinity engine was designed to provide a Diablo-style button mashing and barrel smashing experience on the play side and it does this very well. In fact this is the real heart of all the games made with the engine in a lot of ways, if you liked Diablo, if you like killing things and taking there stuff while pressing the buttons a lot, you will probably really like any game made with this engine and they seem to come out like once a quarter.
On the character creation side, you get 1-3 skill points each level (1 at levels 1-8, 2 at levels 9-18 and 3 at levels 19 and up usually) and each ability costs 1 or 2 skill points to "level up".
Since each character in one of these games typically has 10 powers or abilities, and each ability can rise to level 10 (requiring about 15 skill points), you can see that it's possible to create radically divergent characters.
You also gain HP and mana (called "energy" in the three supers games and Fallout) for rising in level.
There's a few ability scores as well, Stamina or Endurance that determines how many HP you get per level and how fast you recover HP, Focus which determines how much energy or MP you gain per level and how fast it recovers, Defense, how easily you shrug off damage and Fighting, how much hand to hand damage you inflict.
Here's another Diablo "homage" in the infinity engine, if you think about Diablo's four ability scores (Strength, Vitality, Magic and Dexterity) you've more or less got the Infinity Engine down, just sub Defense for Dexterity.
And then of course there's your standard magic items (yes even the superhero games have magic-items) to further enhance your character abilities.
To these already impressive list of customization options that are in every Infinity Engine game, Ultimate Alliance adds a few new ones. Each character in Ultimate Alliance has 4 different costumes, and each costume adds three abilities to the character.
In other words, Captain America has 10 powers that are the same no matter what costume you give him. Things like Shield Throw and Patriotism (that temporarily buffs the entire party).
But he also has 4 costumes, and each costume has three seperate abilities. So if you play "Classic Cap" you'll have a few different wrinkles to add to the character than if you play "WW II Cap" or "Ultimate Cap".
Also you can have radically different teams. There's 20 playable heroes but you can only have four in your party at any given time. Obviously you want a team roster that compliments each other, a ranged attacker like Cap, who also has some party-wide buff abilities, works well with other characters who inflict physical damage. Spider-man, with his ability to web targets and stop them from closing on the group, works well with a ranged specialist and so forth.
Finally your TEAM gains levels, with team levels determined by Reputation rather than XP. Your team can gain abilities just like a character, giving you bonuses to damage, or energy or defense or even XP while those specific four heroes fight together.
And one of the abilities of the "team as character" is to add a "bench", allowing you to add additional members. You can have four players in play at once but sometimes an extra hero is handy when one falls in battle, or for fighting villains who need a certain power (perhaps an all flying team).
So as you can see, on the character side there's a LOT to tweak and min-max to try and form the perfect party and a lot of the heroes can go in different directions. Depending on the powers you take, Iron Man can be a devastating close-in fighter or a skilled sniper blasting enemies from afar with energy attacks.
Compared with OTHER Infinity Engine games, Id rate Ultimate Alliance pretty highly as well. Champions of Norrath, set in the world made famous by the Everquest MMORPG would be the best in my opinion, with Marvel Ultimate Alliance and Dark Alliance II comining in tied for second.
Champions of Norrath II Return to Arms and the two X-Men games were ok... and the original Dark Alliance, along with Fallout are great games but just to short for me to recommend buying them. I would strongly suggest renting them though.
So, there's my exhaustive run-down on the Infinity Engine and its many games.
Hope this helps and that it was an enjoyable read.