Select: Go back and choose another character before starting a stage (useful if you used a revival on your preferred character)
Down & Y does a crouch attack. B, Y is a jump attack, no directional variants, except for Wolverine, who has a dive attack with Down & Y. Iron Man can double jump with B, B. Everyone, except for Wolverine, has a Down, Down/forward, forward & Y move(Hulk has to be close to enemies to perform his). Everyone except for Spider-Man, has a forward, forward & Y move(Iron Man's can only be done when jumping). Spider-Man and Captain America have an Up, Up & Y move(Spider-Man can dive attack by pressing Y during his). Captain America & Iron Man have a Down, Down & Y move. Cap attacks at an angle in the direction you double press. Wolverine & Hulk have forward, forward/Down, Down, Down/forward & Y moves. Spider-Man & Wolverine can stick to vertical walls by jumping into them, and then climb Up or Down. Everyone has a gem power attack with Up, Up/forward, forward, forward/Down, Down & Y, but it requires a gem be equipped, as well as at least one gem power item. It's a screen clearing/boss damaging attack if needed, except in Wolverines case, where it temporarily gives him projectile attacks.
The Infinity Gems, which are slowly gained over the course of the game, play a large part once you do get them. Once equipped they can enhance your characters attributes. For example, one increases speed, so you can make Hulk, the slowest character in the game, faster, or make Spider-Man, the fastest character in the game even faster. One gem increases attack power, so again, you could make Spider-Man, who hits 6 times quickly, now hit harder during the course of his long combo, or make the incredibly strong Hulk hit even harder. It gives the game a nice pseudo customization feel.
Once you start the game, you have the option of choosing a stage, or visiting the Avengers Base. At the Avengers Base your options are getting a password, or training, which is a boss battle against a hologram of any of the games bosses, chosen at random. Training can be good practice for the special moves(except for the gem power move).
When you select a character, you're taken to a screen where you can use items you may have collected, or equip a gem with up to 3 gem power items. The items you can collect(which happens automatically as you touch them) in the stages include yellow cylinders, large and small, for health restoration; blue cylinders, large and small, which can only be used on the item use/equip screen, again, for health restoration; a square with a 'G' in the center, which is the gem power item that allows you to use the gem power attack; and a yellow shield with an 'R' in the middle, which can only be used on the item use/equip screen to revive a fallen character.
Like X-Men: Mutant Apocalypse before it, this is a single plane game. It has moments of platforming, which sometimes has bottomless chasms to deal with. It sometimes has moments where you're running out of air, and need to move quickly, or your health will start to drain if your air runs out. Wolverine and Spider-Man's climbing, and Iron-Man's double jump can lead to new paths during the stages, and sometimes they have no choice. It won't lead to any exclusive boss battles or anything though.
While there are no lives, it's still a more forgiving game than X-Men by virtue of even having the revival items, which means it isn't game over until your final character has fallen. Also the passwords at Avengers Base, to later pick up where you left off.
There are a couple of negative points. End stage bosses have loading times, which can be disorienting. Depending on the boss, when they move, it's possible they'll get a cheap hit, because you won't necessarily know when you can move. The other aspect is that, the entire game you'll be fighting "evil" versions of Marvel heroes, and then some actual villains as bosses. It's a little strange, like the execution a fan game, more than something professionally created. Especially by a company that excels in the genre like Capcom does.
Outside of that, it's fairly improved over the X-Men game, and it allows you to play as 5 of the most popular Marvel characters of all time.