WoW toppled the other MMORPG's when it came because they offered a much different style of online roleplaying. When WoW was new Blizz was pushing the fact that you could actually play solo and do quests without a group which was a lot different than the other games at the time and has now become standard. That one change made the whole game more accessible and allowed people to play on a much more flexible basis. Also Blizz had the highly successful Warcraft world to draw people into. I also think that creating two player factions was a brilliant move. Think about how it unites people in PvP; you know who your friends are and who the enemy is. PvP in games like Conan are just a free for all, everybody is out to get you gank fest.

Any new game that wants to topple WoW now has to contend with WoW's 6 1/2 years of polish and find some way to attract people with either an already popular universe (Star Wars) and/or fix something that WoW doesn't do well that almost everyone who plays WoW wants fixed or offer something even more mainstream oriented to bring in completely new MMORPG players.

In my opinion the next "ground breaking" game will be a polished, story driven, hunting style flexible game similar to WoW but without a holy trinity. Because inevitably no matter what you do with the current system it is just "Genre Style DPS/Tank/Heal Type"
I don't think mechanics really matter though. WoW basically copied the classic fantasy RPG cliches, some that go as far back as Dungeons and Dragons 1st ed and Tolkien and made it work really well. Polish and flexibility are essential but what does WoW lack there? What does WoW do badly? Graphics? I don't think WoW's graphics have hurt it at all. I can still run Cataclysm on my Thinkpad with 16 megs of VRAM. I'm not kidding. It won't do any fancy DX9 stuff but it runs at 30+ fps. That allows a much larger number of people to play who may not want to spend a lot on or know how to buy/build a gaming computer. It brings WoW more into the mainstream. (Gaming computers are cheap to build now with awesome low-mid range cards out but few people seem to know what to get. I know a lot of people who buy a laptop to watch movies and then end up trying WoW).

So what is WoW doing badly that another company could capitalize on and is a franchise like Star Wars popular enough to pull people away from WoW?