Unless you are a fan, just renting this game should satisfy you.
Right off the bat, you will notice that Multiplayer is gone. This takes a lot of the replayability away for people who are content to just go through the single-player once. Although there are training simulators, concept art, and other little collectables for you to find, you can find almost all of them if you really tried; those who don't try can get around half of them. Unless you really want to get all the unlockables and beat the game on all three difficulties, renting it at Blockbuster should be enough for you.
During the start of the single player mission, you're group of heroes consist of Wolverine, Captain America, Spider-Man, and Thor, protecting a S.H.I.E.L.D. helicarrier. However, as you progress, you will go through the Stark Tower, change people in you're group, and create a team; all within one hour (if you got mad skillz lik meh). Eventually, bosses will get "harder" by being ultra cheap and never letting you attack, although the blue and red orbs that they drop will heal you so that you won't die that easily. The A.I. for your team isn't bad, but could use some improvements. You're teamates might jump off of a cliff trying to kill an enemy you just threw down. You're allies also sometimes just stands there until you give them an order. The cinematics aren't real time, so they usually repeat what you have already learned talking to NPCs. Scripted events seem to be off as well. Sometimes, you will talk to NPCs and have no clue what they are talking about. Then, as you are walking out the door, the scripted event occurs and explains what you have missed. Problems with A.I. and scripted events really aren't that rare, but aren't that common that it becomes frustrating.
However, it really isn't the gameplay that will shine, although the gameplay isn't bad. You're goal can be summarized like this, kill this, destroy this, activate that. No, what really shines is the amount of the Marvel Universe that is actually incorporated into the game. The game has many, MANY places, characters, items, and events that will immediately knock-out any fan. It will even impress people that just know the name of some popular heroes and teams. This huge chunk of the Marvel universe really makes this game playable.
The Wii version, as we already touched upon, really isn't as good as the other versions. The controls take some getting use to if you aren't familiar with the standard Wii gestures. However, the Wii version does let you use certain powers without having to scroll down and select it, a real life-saver is some places. Multiplayer just doesn't seem to be Big N's strong point, and is not incorporated with the Wii version of MUA.
Overall, a good game. Definately not "great", but enough for a rent for casual Marvel fans and a buy for more hardcore Marvel fans.