I was a huge WoW player for a few years. I don't miss the game, but I miss Northrend and how Dalaran was in late Wrath. Leading me to suspect that a lot of it is not so much the game but who you are while you're playing it and who you're playing it with -- and the thing about WoW is that everyone plays it, so.
Can't stress the bolded part enough.
It's why I left the game I was playing before I tried out EVE. It's why I have been having fun playing STO lately, and it's why I still play EVE.
Even when I run out of things to 'do' ingame, there's still people to interact with, and sometimes that results in more things to do, or reasons to do already-done-to-death stuff again.
I guess that's why I've always had so much fun in WoW. I tend to be pretty helpful as a player and I've met a ton of people through WoW's RP scene that I became friends with before I raided or PVPed with them. I've never run out of things to do, even after running through PVP content in the last expansion and PVE content in this, interspersed with that damn pet battling game and all the achievement running. I've always had RP vying for my time there and a lot of friends who did things like ran a biker gang run across the Eastern Kingdoms for S&Gs. We've been able to drop into bars and start brawls, had a pool party in Teldrassil, and more.
It's why, even though I've sort of been recalled, though, I still stick around here and try to log into EVE every now and then. I may not be as heavy of an EVE player and probably never will be, but the point of an MMO is people. I've made friends here and like the RP scene well enough, so I'm probably not bound to drop off the face of EVE yet. One of my brothers still plays EVE primarily (he's the reason I play in-game at all), so I drop in to help him out.
But when it comes down to it, even my wife loves WoW. The WoW players in my immediate family form a ten man at least once or twice a month. I've not only traveled to meet people I've met in WoW, I've had them live with me for a week at points until they get back on their feet, helped them get jobs, been helped myself when I've needed help. Those people are really my friends, often better than my flesh and blood ones...
.... who also play WoW. It's sometimes inescapable.