If hanging back and unleashing projectile attacks is your thing then you might want to enlist Spider-Woman or Captain America. Players that want agile hand-to-hand combatants will dig playing as Deadpool or Elektra. Whatever your style is, you can pick the characters you like and power up your preferred attacks through the game's diverse power-up system.
Like its predecessors, there are tons of unlockable features. Each character has multiple skins, some of which give stat boosts or enable new attacks. Playable comic-book missions for single characters do a nice job at breaking up the team gameplay. Various action figures are strewn about each level, allowing you to unlock new characters. There are also more obscure ways to unlock new heroes; they might be tough to find, but the payoff is absolutely worth it if you're into comics.
So you've got tons of heroes and the series' stock gameplay. These ingredients are mixed into a well-written story that's brought to life with excellent voice acting. The heroes are up against Doctor Doom in an adventure that leads them from the S.H.I.E.L.D. helicarrier to Atlantis to Mephisto's Hell to Asgard to a Shi'Ar mothership to the Skrull homeworld and more. The cutscenes can feel a bit stilted, but they're used well as a storytelling tool and do a fine job of making the game feel like a comic-book crossover-event.