Ok first of all I have to admit that City of Heroes was probably my first real MMO other than Runescape so, I'm a bit biased in favor of it, even though I only played it for two to three months.
It really is a fun game though, if you are new and don't mind grinding or slightly rough death penalties or certain other things. Without ado here's my top ten:
1. Fast and Fun Grouping! City Of Heroes was far and away the best game to date that I have found for finding groups and getting new members to you and getting to where the fun is, all the time. Everything in the game lends itself to this better than the more archaic Everquest/WoW formula of "figure it out on your own and make all means of teleporting people to your party more difficult than they need to be" Individuals don't need assistance to teleport others and it costs nothing other than power, which regens really fast.
2. Dynamic Group Sizes! Dynamic Group Dungeons! City of Heroes was sheer genius, actually its just common sense applied by people who love fun instead of who love to make stuff easy to make. You can have as many people as you want in your group! or as few, and you can still run dungeons! And the dungeons change based on how many and what the average level is too. Genius. Usually this amounts to a moderately to highly challenging instance with good to great exp rewards and neat boss drops. Why hasn't this been done? Many claim in a more loot based world its difficult to get right but we've already seen that that is bull. I personally think its just cuz nobody wants to bother. They are too comfortable with their previous system and are "stuck on the rails" of their old ways, so to speak. Learn from City of Heroes, don't just let it be the only one people, plz.
3. Really cool missions/dungeons/task forces etc.. Some of the dungeons and missions are very very unique and took a lot of thought and are not just killing trash mobs and yadda yadda, they have timers, they have some tactics, and strategies you have to implement, and story line, there's lots ot love here. Doing them over a second time isn't always da bomb, but some ARE fun to play over and over again.
4. Unique character builds Not only can you make a character right from the get go that is unique looking, you can make them have unique, almost entirely unique in abilities, thanks to the primary, secondary and choose your own adventure power set system, plus the abiltiy to tie different weights to it via slots every other level. This was customizing genius in a can. This makes the point system of WoW look like a Diablo 2 cop-out.. oh wait, it is. Well it makes Lotro's system look pretty weak also, and most other games too. Why not choose your own powers as you would in Morrowind or other such games? I see no problem.
5. Cool areas and ambience -- Some might argue with me on this one, bu I thought the areas were really neat. Yes they were a little heavy on the run down cityscape, some were just gone-to-seed industrial areas, others were more up to date sections. Well the game is about a City of Heroes right? So why not. still the change of pace of Perez Park and the Hollows and the Pier and stuff and islands was neat and cool, as was the fact that there's a vile secret sewer system under the city connecting the zones and more crap to explore than you can do in the amount of time I was playing. (Not to mention Alien invaded areas and such) I loved the public transit systems and a number of other nice touches, as well as the waves of guys who attacked in the hazard zones.
6. Usefulness of archetypes -- All archetypes are useful, but seldom is any particular one absolutely essential, sometimes tankers or defenders were needed but really, you can get by without them if you are careful, and bubble up properly, it just takes longer.
7. Badges are fun! -- OK well, it was a cute idea to put badges into the game and this was the first time anyone had done such a thing and Blizzard ignored it but Turbine learned from it at least and then later Blizzard copied it and lo and behold people like to collect all the Pokemon, err badges...
8. Colors! Flashy Powers! Boom, bam bang! - I just loved the look and feel of being powerful and knocking people senseless, it comes with the territory in a superhero game but I love it regardless.
9. Flight, super speed, super jump - This game had the best ways to get around yet implemented in ANY MMO. I yearned for super speed in WoW and Lotro and WAR more times than I can count. Super Jump was great for us explorer types, as was flight, though much too slow unless you enhanced it.
9.5 Targeting of destinations -- every game needs this, Ok well at least they should consider it. If you pick a destination or quest, City of Heroes gives you a pointer in 3D space to where you need to go to do that. Soooo handy
10. Champions Online -- Thankfully, the same company who made City of Heroes is coming out with a better one and instead of naming it City of Heroes 2, it is Champions Online. I will definitely be trying it out.