WoW-killer? absolutely not. Nothing is going to "kill WoW" except WoW itself. I refer you to Extra Credits for more on that.
Exhibit A: The Future of MMOs [http://www.escapistmagazine.com/videos/view/extra-credits/1906-The-Future-of-MMOs]
Exhibit B: The Skinner Box [http://www.escapistmagazine.com/videos/view/extra-credits/2487-The-Skinner-Box]
Is the game going to have an impact? Well, that remains to be seen. A lot of people seem to be having fun with the betas, but are not quite blown away by it. RIFT has potential, and from a fledgling studio that's saying a lot. A great many new ideas (supposedly) are implemented, and an effort to break the mold of "WoW-clone" (or Ultima-clone). Unfortunately, nearly everything I've seen about RIFT has it completely outclassed by the upcoming Guild Wars 2. Sadly, I'll be buying that instead.
RIFT is an excellent first try and, if they're not ruined by lackluster sales/subs, I expect to see great things from Trion in the future. They have as good a chance as any to succeed, and I hope they do.