First of all, let's talk about the good things
As I said at the opening of this post, I honestly do believe that WoW did many good things for the genre. It was the first breakout success for this industry for a series of good reasons that even the haters have to agree on.
When it comes to the quality of the experience, Warcraft did it well. Any machine can run WoW and run it well without significantly detracting from the game's colorful and engaging aesthetic. The artwork behind the game is well drawn, the ability icons that go in the hotbar are attractive and easy to understand at a quick look, the spell effects and animations are attractive, and the overall visual appearance is sound.
The quest lines, while some are very mundane, were a breakout idea that really wasn't so breakout. Blizzard didn't invent the questline as much as they did refine the idea to produce something that's easily understood by everyone. If you see the marker over someone's head, it means that they have something for you to do. Other games had hid this type of gameplay mechanic by simply not directing the user to it. If you wanted a quest, then you had to find it by talking with all of the NPCs around you, which could become very overwhelming.
The quests themselves are solo friendly (a first for an MMO) and support bite-sized gameplay sessions. You can drop in, get a few quests done and feel accomplished, and then drop back out without having to wait for a group or only be able to do one quest and get a boring reward.
Even the way the game is paced, with your character moving from zone to zone or being directed via quests to the next set of hunting grounds makes sense. This type of stuff easily leads the player the right way without being annoyingly overt or too confusing to understand. Sometimes you just go follow the quests without even realizing that you're actually being controlled by the game design. It gave you the sense that you were traveling to help others and fix the wrongs in the world. It felt smooth, easily understandable, and it worked.
For all of these reasons, and for reasons I didn't even list like the quality of endgame content and the quality of some of the encounters, the game became "the new thing" for many, many gamers. Very, very few of my friends have never touched World of Warcraft, and many of my friends are not active gamers. That says something.
Yet there are certainly bad things...
When I say there are bad things about World of Warcraft, I should really refine what I'm trying to say. World of Warcraft is not a bad game, it's actually a great game, but it's mere existence has exerted a very odd influence on our industry.
After World of Warcraft garnered millions of subscribers, started a cultural trend of Horde versus Alliance, and re-designed how MMO developers approach game design, the industry seemed to snap into a very clear zeitgeist. What I mean here is that we've become mired in our own success.
There are three clear things that World of Warcraft has done to corrupt our genre of gaming, and they would be the "It's Not WoW" syndrome, "It's Too Much Like WoW" syndrome, and "The Million Subscriber Success" effect.