According to former Blizzard chief creative officer Rob Pardo (so he's biased but he's still quite correct), World of Warcraft killed the "MMO" as a descriptive category:
Speaking to Develop at the recent Games First Helsinki event, Pardo said massively multiplayer online games have expanded and evolved away from how people used to describe them. He said following the runaway success ofWorld of Warcraft after its launch in 2004, a game that still boasts some 7m users to this day, a wave of companies tried to copy the winning formula. Not one of these were able to replicate the same level of success, however... “If anything, I think people are even avoiding the term MMO. A really good example is Destiny. It clearly is an MMO. But they’re really trying to avoid calling it that, and obviously it is a very different type of game. But I think that’s a good example of how with MMOs, the term has been eliminated. But you kind of continue to see the influence in games that are persistent world games that have spawned out of that. It’s just people seem to avoid the term MMO now.”
Even better than Destiny, I'd say Day Z or Minecraft are examples of MMOs or multiplayer games with MMO's best features that aren't generally called MMOs. (For that matter, League of Legends, a multiplayer fantasy strategy game, is not an MMO and is even more popular than World of Warcraft.
There's a lesson here for Project Sansar and High Fidelity, and other "virtual worlds" (as they're usually called) which are sometimes described as MMOs (since that's their closest cousin):