Unlike Sanctum, it's got persistent stats and items for each character, and four character classes to choose from. It's also got free-for-all loot including basic coinage and it's not the easiest thing in the world to trade -- you have to drop the item on the ground (somehow) and let the other person pick it up, while you're in a game. You have to spend mana dropped on a level to build towers, but oh look instead you can put it into your personal account and use it to upgrade or buy items! I would NOT want to play this game with random strangers, it seems like it's designed for griefing.
But then, I'm not planning to. I hardly ever play anything but city of heroes (and other MMOs when I'm playing them) with random strangers.
Anyway, there are four classes and up to four people on a map, so in theory each person can pick one, but I'm not sure that's really a good idea because their utility is... not... similar. Let's compare:
Summons a Fireball tower that does ~400 damage in an AoE every 3 seconds. Enemies have to beat on it for a while in order to kill it. Also can summon a very cheap, fairly tough wall in front of its fireball tower that the enemies have to kill first. Primary attack is a rapid-fire ranged shot (as fast as you can click the button) that can be charged up to do an aoe, secondary attack is a melee-range pbaoe with knockback.
Hunter or whatever the frack it's called; the girl with the bow:
Summons a stick of dynamite that does ~400 damage in an AoE every 12 seconds, a total of two times, and then is gone and has to be rebuilt. Multiple dynamite sticks have to be spaced far apart or they'll set each other off uselessly. Can also summon a kind of expensive gas trap that stuns enemies for about half its reset time, that also has two charges. Primary attack is a kind of slow single-target ranged shot (two attacks per second generally, but you can hold down the button), secondary attack reloads your weapon (taking FOR-FRICKING-EVER) after you run out of ammo.
... seriously. There's just no comparison. Oh, and I didn't even mention the two melee archetypes (because I didn't play them myself) which are generally considered to be hideously overpowered compared to the ranged ones (although the apprentice does have the best towers) (according to random forum posts; I've only played the game a couple times).
Still, it's different from Sanctum, and fun, and maybe Rowyn will stop complaining about never getting loot if we make sure to leave some for her. c.c Sorry!