I think it's important to understand what Blizzard actually did with WoW, though. As others have said, Blizzard as a company has never really been about innovation, but rather about taking all the little things that make other games successful then combining into a well-polished, fun-as-hell game. This analogy is a bit of a stretch, but I'll make it anyway: Shakespeare never wrote an original storyline in his life. Virtually all of his plays come from appropriating source material (histories, classical myths, etc.) and he was prone to outright plagiarism every once in a while when it suited him (Cleopatra's barge description, for example). But does it matter? He's freaking Shakespeare, it's all in the execution and in the performance.
Lots of games have a few brilliant little parts to them, but most of the time those parts are overshadowed by serious game flaws. Blizzard, however, has the capital and the development to ensure that everything in the game is well-polished as well as humanly possible. Their games, pretty much across the board, are fun, relatively easy to learn, and have incredible replayability, and I do think they really have raised the bar significantly for the MMO genre.
A lot of these changes have resulted in a bit of a dumbing down of the genre, but I think a lot of that is actually a good thing. It makes MMOs attractive to investors so we're more likely to get good games in the future once people see the kinds of cash Blizzard is raking in (for some reason, I always imagine Blizzard'd CEO swimming in piles of cash like Scrooge McDuck). I played EQ at release and no matter how nostalgic I get for those good old days, I can't say I miss the de-leveling, cross-continent corpse runs because I died before I was able to get a bind, and 18-hour farm fests in ToV while competing with another guild for spawns, trying to train them before they trained us. I quit WoW and sold my account a few months ago (for 800 bucks, no less), but I recently bought a new account and have been messing around with that because the game is still pretty fun.
I ended up quitting because the grind just became too much; I was in a BWL/AQ40 farming guild before Naxx came out, but the size of that freaking dungeon and the amount of time necessary to progress finally broke me as the game began to revolve entirely around grinds that got even worse with TBC. The PvP system overhaul was great, but that became another grind that you either have to do exclusively from PvE raiding or spend 40 hours a week doing like it's another job. If a lot of the game content ended up dumbed down on the lower end, at the higher end it just demands too much time to be even remotely competitive. Starting a new PvP character, for example, you now need to grind out an entire season's worth of Season 1 Arena gear in the BGs before you can even think about doing Arena.
If WoW pushed the game too far into the mainstream and dumbed it down, I think a few of the games on the horizon stand a good chance at cracking into their fanbase, with Age of Conan the most interesting for me. Warhammer Online looks good and the one I'm really, really excited about is the no-info Bioware sci-fi game, and I've also read that Tim Cain, the lead dev for the Fallout series is working on a sci-fi MMO for NCSoft, although I haven't seen any more into than that it exists.