
So, for me:
1. City of Heros
2. 5 years and a bit, letting my account lapse in June 2012 (haven't played since January)
3. I'd like to qualify my answer with why I started playing CoX in the first place. My wife had been accepted to veterinary school, and I suddenly found myself a vet school widower. I found i needed something to keep myself busy during her constant studying, class work, etc. I came to CoX then looking for an evening social distraction and landed on Virtue due to having some minor aquaintances on that server
a) CoX had a very fast learning curve. And I came to CoX after it had a fully fleshed chat system and was able to find an rp super group within a few weeks that met my interests. The community on Virtue was mostly light to moderate rp'ers, and by and large a pretty friendly group towards n00bs. That element of social community really made CoX a game easy for me to play for years. It certainly wasn't the game mechanics, as CoX is an MMO that is by design focused on casual players.

c) I came to CoX just before the Cimerora expansion (Issue 11 I think). Which means there was already 4-5 years of content in the game. And NCSoft has been actually pretty good about continuing content release since I started playing. As a casual player it took me 2-3 years just to work through the base pre-Going Rogue paid expansion material for City of Heros and City of Villians respectively. I didn't even join the task force/raiding community until my 3rd year of playing.
These three elements: 1) a social, (mostly) welcoming community, 2) replayability, and 3) content addition/creation. These are the things that kept me paying NCSoft for what will have been 5 1/2 years when my prepaid time expires next month. Now, why did I leave? I've already mentioned that I accept fatigue/boredom as a legitimate reason, and that was definitely a part. I'd done everthing that I found I wanted to. But the move by NCSoft to that horrid, raid-focussed Incarnate system just killed it for me. I am very much a solo/small group PvE kinda guy, and NCSoft has by and large abandonned that element of CoX.
Brian