I used to know basically no gamers who didn't play WoW. Now I don't know that I still know any. I was one of the loud defenders of Blizzard's choice to enter into a business merger with Activision, and I have been forced to admit that I was wrong. Blizzard's handling of events since then has been spectacularly bad -- I left over the Real ID stuff, myself. (Yes, I know, lots of people say they "backed down". Only temporarily and from the most ridiculously stupid parts; many other aspects are still horrible now, and some of the bad ideas they postponed may come back.)
Thing is, in MMOs, network effects are king. If you want to play a game with your friends, the game your friends play wins. But once you start losing that "everyone I know plays X" spot, there's not really any particularly great technical advantages of WoW over a lot of other MMOs, and quite a few are in many ways better. Even apart from my personal grudge against Blizzard, I found other games to do a better job of things that mattered to me, and I really got sick of Blizzard's active hostility to various parts of their user base. It was a real eye-opener when, after Blizzard spent several years explaining that it could never be possible to tweak the rulesets between PvE and PvP servers, Trion turned around and did it in a week during the Rift beta.
So now I play whatever I happen to know other people who play. And none of the individual games have the population density WoW did, but I am not totally unhappy about that, because it means more choice and more selection.
Stuff that's still going:
DDO: Very different philosophy and design, pretty cool. Overall I'm pretty happy with how Turbine runs things. The microtransaction stuff isn't as intrusive as I thought it would be, and the game design has some really nice appeal.
Rift: As a game, this is basically what I always wished Blizzard would do, and then some. Developers have been pretty responsive to user feedback, and there's a lot less of a focus on tedious time sinks. Big weakness, from my point of view, is that there's been basically no visible community maintenance in ages, so not only are there people who engage in massive, long-term harassment and abuse, but now there are lots more people who are abusive because they're convinced they can get away with it. Still, if you just wanna play with a few friends and ignore public channels, the game itself is amazing. (Slashdotters may care more than others: The addon API is beautiful. One of the nicest development APIs I've used.)
TSW: Not hugely happy with Funcom, but the game is fascinating, and does a lot of things which are radically different from other MMOs, some in very interesting ways. Also pretty responsive to user feedback in a lot of ways.