Heres, the difference. Wow is an all ages MMO infested with stupid little kids and 1337 haxxors. Age of Conan is an M rated MMO that has blood, fatalities and boobs, and is technically only for 18+. Also WoW is very very cartoony with dated graphics, while Age of Conan will force you to have a halfway decent rig to play it with any bit of performance.
It wont kill WoW, but it will definately pull alot of the more mature gamers away from it.