You simply don't understand what you are missing if you haven't played this game.
If you are used to the rest of MMOs, which are all essentially carbon copies of WoW - then this will truly be a breath of fresh air. The very dynamics and feel of City of Heroes are unique, which is something that cannot be said for any other MMO that I have played. It has a very "clean" element to it that perfectionists and people with OCD will greatly appreciate. All in all it is a very shiny game, which makes it very surprising that it has evaded detection from the mainstream's eye for all these years. It feels like a "real game", unlike any other MMO. Alongside that, it is PACKED full of so much content, that in the literal sense - you will never experience it in a lifetime. I myself have played for more than Three Years (That's not how long I've had an account, that's the TOTAL TIME that I've spent playing the game since I joined over 7 years ago), and I have yet to see it all.
The game is constantly being updated with MAJOR free updates, which include much more content than that of your typical MMO update. Recent additions include things such as allowing you to change the animations and colors of your powers, the ability to create your own custom missions (or quests as they are known in the mainstream), and even a special system that lets you benefit from time spent logged out of the game. If that's not enough for you, the developers are also constantly adding new Zones and Enemy Groups (and every time they add new content, it becomes available to unlock for use in the "Mission Architect" as it is called).
City of Heroes replaces xp loss with the "XP Debt" system, which is not a completely unique concept. Instead of going backwards in xp when you die, half of your xp gained goes to paying off your xp debt. Tired of popping up in a graveyard near the same enemies that put you there? In CoH, you are resuscitated safely inside the local hospital, with plenty of protection on the outside from the various police forces. But don't let that make you feel entirely safe, because the game includes various "Hazard Zones", where you'll find yourself navigating fractured sections of earth, nuclear power plants, and other dangerous areas inhabited by deadly forces. What's more is the random Invasions. Yes, that's right - invasions. You might just be calmly standing by when hordes of alien dropships pour over the city. But that's not the only type of invasion. Others include Zombies, Werewolves, and a unique juggernaut-like alien race called the "Hamidon".
As far as epic goes, few come close to the level that City of Heroes has reached. You can join a team of 20 or so, and attempt to defeat giant monsters. Some are right out in open zones (Such as the Giant Squid Monster, "Lusca", who dwells in the waters of Independence Port), while others have to be tracked down in massive labyrinths (The Kraken, for example, can be found deep within an abandoned section of the extensive sewer network).
City of Heroes also has more instanced indoor missions than any other MMO that I have encountered, effectively eliminating the pains associated with Kill Stealers. Another great thing about the missions in the game are the fact that you can play one mission, and get on with your life. It's really the only MMO that won't dominate your life (unless you let it of course, which can be said for any addicting game).
Since this was the first Superhero based MMORPG (released in 2004), it remains by far the best one out there. Competitors such as "Champions Online", are almost laughable by the standards of a CoH fan. Though it's not for everyone, especially those who are used to the dynamics of games like World of Warcraft. Much of the game works differently, and it might leave you a bit confused at first. You don't change your looks by finding special "Armor" and such things, but rather by Flying, Leaping, Speeding, Teleporting, or walking to a place called "Icon", which is a store (not a real store, you spend in-game money - not real life money), and speaking to a tailor. You can have up to five costumes, with which you can switch between at will (you can even make one of them into civilian clothing, or have your character turn into a monster). The greatest thing about CoH is the fact that all of the content in the game is avaliable to EVERYONE. Many MMOs will make you pay seperate fees to acquire certain in-game items, but not this game. If you don't have it, it can be unlocked by defeating some type of enemy, reaching a certain level, or completing a certain quest. Though there are the occasional rare items which you can only recieve during certain events.
The events are in their own right, just as awesome as everything else. The entire game world changes on major holidays such as xmas and halloween (especially halloween, in which the sky is covered in darkness and hordes of zombies, witches, werewolves, and GIANT pumpkinheads appear at alarming rates). During the halloween event (which is my personal favorite), you can go trick-or-treating in the city, clicking on almost any door. Randomly it will give you a "trick" or a "treat". If you get a trick, enemies pour out of the door you clicked and instantly attack you. If you get a treat, you get a random prize of anyting from a quick powerup to rare salvage that you can use to craft special items. There is also a chance for one of four special halloween salvages, and when all four are given to a special contact - you unlock an extra costume slot! Finally, there are tons of special costumes that act as powers or abilites, and when you click on them you turn into one of the many enemy NPCs in the game.
There is just too much of this game to explain, and so the only way you can really experience it for yourself - is to check out the trial version on www.cityofheroes.com
Ask any of the veteran players any questions you may have about the game when you get in. Who knows, you might just bump into me.