Marvel Heroes is a massively multi-player online (MMO) action role-playing game (ARPG). Short translation: it's an MMORPG where the vast majority of what you do is combat. There is a story-line, but it's fairly shallow and not character-specific. The main attraction of the game (for me, at least) is that you can play as many different characters from the Marvel universe.
Unlike almost every other ARPG that I've ever played, characters in Marvel Heroes aren't based on a race or class. Instead, each of your playable characters is a personality from the Marvel universe (i.e. Iron Man, Spider Man, Captain America). There's a ton to choose from - 53 at the time of this post. You choose one when you first pick up the game, and can unlock others through various means: purchase, picking up in-game currency, occasional giveaways and rewards. You choose gear and decide which abilities to focus on for each character, so they're not always the same: your Jean Gray might be focused on her psionic abilities, while my Jean Gray uses her Phoenix powers. Both thematic for the character, but the player chooses how to customize their version.
Marvel Heroes uses a top-down isometic view, so you're always looking down at your character moving around the world. That's important, because there's always a ton of things going on as you fight your way through hordes of villains. A first-person viewpoint wouldn't have much of a chance of keeping up. Most combat is an explosion of lots of different superpower effects, to the point where it's difficult to see what all is going on. The interface provides some help, like pointing out where a boss is among all the chaos, but it can still be a challenge to keep track of everything that's going on. And that's part of the fun...the "action" part of ARPG is certainly front and center most of the time.
My experience with Marvel Heroes has gone through three major phases: discovery, expansion, and end-game. They overlap somewhat, but I think every player is likely to follow more or less the same path.
When a player first starts Marvel Heroes, everything is new, thus the Discovery phase. You've got just one character to play, and you take them through the story chapters. The story is pretty simple and feels very much like a comic book, especially since it is told largely through "motion comics" - basically slideshows using comic-book style art with voiceovers. You get to see all kinds of locations familiar to Marvel universe fans, from Hell's Kitchen to the Savage Land to Odinheim. You'll also be introduced to the concept of patrol zones (open areas with tons of enemies to beat on and periodic villain invasions) and terminals (repeatable areas with one major boss and his minions).
As you make your way through the story and have seen most of the various content areas, the experience becomes about Expansion. The most obvious form is unlocking new characters to play, which can be done quickly with purchases from the store or more slowly through collecting gameplay items. I've spent very little on character unlocks, myself, but I know people who have bought quite a few. My purchases have mostly been unlocking new storage space, and once a costume (90s X-Men Storm...my favorite character from that old cartoon). You can also improve your access to various items and abilities by leveling up the crafter and enchanter NPCs, obtaining influence with the Genosha Liberation Front, and collecting items for the various weekly event vendors. Then there are Team-Up characters, who are unlockable like playable characters. You can choose one Team-Up for each of your playable characters, granting various buffs and some combat assistance.
The expansion never really ends, but once you've gotten a few characters up to the maximum level of 60 and improved their gear/builds, you can shift focus to the End-Game. There are several raid zones meant for 10 players, and special "cosmic" versions of the patrol zones only accessible after your character completes a moderately-difficult trial zone. The game feels very different with a character at 60, as difficulty scales up and more zones are available to access.
The Marvel Heroes experience is almost entirely about the different playable characters, Once you've gone through that initial discovery phase, you won't find much new in terms of story or areas to explore. The game still comes down to repeating the same areas with the same enemies, but using different characters keeps it interesting. The game reinforces this by giving each character their own personality, with different one-liners during combat and occasional comments as you pass through different areas or near other characters. (For instance, my Storm likes to compliment every Psylocke she sees, while Emma Frost has a put-down for almost everyone she runs across.) If you get multiple characters up to level 60, each adds an experience boost to all your other characters, which smooths out the slow grind of getting a new character through lower-level content.
I don't play Marvel Heroes every day (though I'll usually log in once to claim daily rewards), but it's enough fun to keep me coming back on a semi-regular basis. Especially as the developers continue to add new playable characters, additional areas to explore, and special events.