First, the gameplay is just fun. Different characters are given different abilities and attacks. For example, Ms. Marvel can fly, Cyclops can attack from a range, and Daredevil can double jump. The character's catch phrases wear a little thin but they're unique to their own personalities and character histories such as the Thing mentioning growing up on Yancy Street.
Another reason this game is great is the character selection. Some of the characters are pretty obscure, such as the Falcon or the Valkyrie. Some, like Spider-Girl, are actually not even from the main Marvel comics continuity. Some characters are given different versions such as the Hulk, Red Hulk, and Gladiator Hulk. They even pull some things out of the Marvel continuity vault like Spider-Man's clone, the Scarlet Spider.
The game also has surprising depth. The game features lots of side missions and mini-games. You could see how different players would be attracted to the game for one feature or another.
Although there's a pay feature it isn't necessary to access any game feature which is nice.
So I'm impressed with this game - it's my new addiction! Way to go Marvel!