I found a recent post at Don't Fear the Mutant and the comments that followed really thought provoking. In certain corners of the web there is a running meme that goes along the lines of "Every major MMO since World of Warcraft launched has been a WoW clone, WoW murdered innovation in the MMO genre. " Of course this relies on defining every MMO that has classes, quests, levels, and turn based combat (i.e., the great bulk of CRPGs, online or off...and a lot of PnP RPGs as well) as a "WoW clone." However, let's just accept that lunacy at face value and move on.
The basic problem with this meme is it's myopic focus on big hype big budget MMOs. A lot of MMO commentators seemingly only pay attention to the MMOs that have the biggest development and advertising budgets. Yes, indeed, the really big budget MMOs to tend to be Everquest style games with classes/ levels/ turn based combat, and have a lot of design similarities with WoW. Like WoW they are usually solo friendly, have a relatively easy to understand class/ level based advancement system, include a lot of quests, and have a UI very similar to the one pioneered by Asheron's Call 2.
However, seeing these these features in a big budget MMO should not surprise anyone because such games are by definition aimed at the mass market. Everquest was the most financially successful MMO of the first generation, and World of Warcraft was the biggest success of the next. If you are designing an MMO budgeted for more than 100K players, you'd be pretty stupid to ignore the mechanical similarities of the most successful past MMOs. And it is for this very reason that big budget MMOs don't count when evaluating whether the MMO genre is stagnating. No one sane is going to put a WoW budget behind a niche / experimental game design.
As soon as you take a single step away from the mainstream, you can see that there are actually a lot of successful post WoW MMOs that bear little resemblance to it. For example Dungeons and Dragons Online has the deepest character generation system this side of EVE, real time combat, and a guild leveling system more involved than anything I've seen elsewhere. It's also the third most popular MMO in North America according to the NPD. Wizard 101 is doing well enough to put out a new world roughly every six months. The card based combat, pho Harry Potter setting, and mini-game method of mana regeneration draw absolutely nothing from WoW, save perhaps bright colors. Puzzle Pirates is also doing quite well and has as little in common with WoW as I can imagine and still technically be an MMO.
Take two or three steps off of mainstream into the really niche products [and in many cases not financially successful at all, I will allow], and you will find even more innovative MMOs. For example Myst Online URU, a completely free to play MMO where there is literally nothing to do except solve puzzles and explore. Or A Tale in the Desert, where as far as I can tell you mainly craft. Or The Endless Forest. You play a deer. There is no combat, crafting, or even chatting. You communicate with others using only emotes.
Just because the only MMOs someone pays attention to are all DIKU/ WoWish doesn't mean that WoW has ruined the genre. It means that some MMO commentators need to broaden their horizons. Innovation continues to occur, and even be rewarded financially on occasion. When I see commentators say "WoW ruined MMOs", what I mainly hear is "no developer has been willing to put a huge budget behind a niche product that caters to my peculiar tastes."