City of Heroes is defiantly the most addictive MMO I've ever played. And I've played a lot.
First off, character customization. Unless you plan to make to dress like someone else in the game, odds are you won't find anyone else that looks like you. There is so much character customization, it's not even funny. CoH defiantly gets points for this.
Once you get in the game, though, players will except missions and fight gangs, cults, mutated creatures, robots, zombies and who knows what else. Missions usually have you going into a warehouse or office and fighting all the enemies in there. Maybe disarming a bomb along the way. Either that or you hit the streets and take down a few baddies harassing the citizens. The missions in CoH vary and are fun and quick to complete. There are also Task Force. Task Force are a chain of missions you complete in a team of eight players. They usually take a large amount of time, but after you win, you are rewarded with a badge saying that you have one and a high level enhancement.
Enhancements are things that are used to increase the power of your...er...powers. Inspirations are used to give you a quick health boost and such, like a potion would in games such as WoW. Influence is the currency in CoH. It is used to purchase enhancements and inspirations. It can also be used at Icon. Icon is a tailor store where you can change you costume after you've created your character.
And finally, powers. Every time you level up you have to go a trainer. Trainers let you learn more powers every other level. At the other level, you gain more slots to put enhancements in. You have your main powers and your secondary powers, and they vary as to what you are(scrapper, tanker, defender, controller or blaster)
All in all, CoH is very addictive and easy to play. I suggest it to anyone willing to play an MMO. Good luck, and have fun in Paragon City!