It won't be a WoW killer.
It will happen similar to Rift.
It will drag some people away from WoW, but the vast majority won't leave WoW for it.
Most people are too commited to WoW to drop it and start playing a new game.

As for D3, I dunno if it will be either. That real money auction house they announced is a terrible idea to be honest. Terrible fomr our perspective that is. From Bliz's it's great.