City of Heroes is a strange sort of MMORPG. You play as a super hero, and after spending hours creating your character, choosing what kind of hero you are, and additionally choosing your primary and secondary poweset, things develop a bit differently than you may be used to.
(Each Archetype (One might call it a class) can be customized further using "powersets." For instance, the hero archetype, "Blaster", can choose from Assault Rifle, Archery, Fire, Ice, etc primary powersets, along with various other secondary playstyles. While your archetype determines the general playstyle for your character (Blasters do ranged damage, Defenders do amazing party buffs, Tankers tank, Controllers, well, control, etc) your powersets determine how you go about doing this. The only skills common to all characters are bash (a weak melee attack), Sprint (Move speed buff that you will use constantly) and rest (heals you). Every other skill you acquire (with a few exceptions) is from your chosen powerset.
At level 5 or 6, I forget, you gain access to additional powersets outside of your primary and secondary sets. These additional sets contain various abilities, but most notably, the contain travel skills. These are things like flight (fairly slow, but very safe), super speed (fast, relatively safe, but hard to control), super jumping (fast, versatile, but dangerous), and teleport (various issues apply that require some knowledge of the game).
At higher levels you can aquire further SUPER super powers of various disciplines.
Your inventory will be the first thing you notice is very different. You have three kinds of inventory - Invention, Inspiration, and Enhancements. Invention holds "invention salvage" and recipes, and is used exclusively for making costume pieces (like wings, which normally aren't available) and enhancements. Inspirations are sort of like disposable spells. They do various temporary things like heal, increase your armor, increase damage, so on and so forth, and are consumed upon use. Blasters eat these like candy. Enhancements can be slotted to your powers, kind of like materia, to make a reference to something that's likely familiar to you all. They increase things within that power such as damage, accuracy, so on and so forth.
Experiance is gained like any other MMORPG. Killing bad guys and completing missions earns you experiance. Get enough and you gain a level.
Combat is half familiar, half new. For one thing, you don't have an auto attack per se, but any power on your power bar can be set to auto-activate when it recharges. Things like the ability to fly come in to play her as well.
Really, it's hard to describe. It's not hard to get a 10-14 day trial, and I highly recommend that. In fact, send me your email address and I will invite you (You get a free pass with the invite), so if you buy the game I get a free month :D
I should also point out that, in my experiance, City of Heroes is more fun than City of Villains. Villains are much more, let's say, excentric to play, and may not be what you're expecting. Right now though you can get the Good vs Evil box set that for like $25 that give you both games, and your $15 subscription fee covers both.
JUST LET ME INVITE YOU SO I GET FREE STUFF