Honestly, I've played almost every MMO that's claimed to be a WoW-killer. In contrast, I was huge into WoW until MoP, and haven't played since.
I don't think ESO will "kill" WoW, but I think it has the potential to do well. GW2 was said to be the one to ruin WoW, but both are going strong with dedicated fanbases. ESO will just be another share of the market, unless it tanks like SW:ToR.
...but SW:ToR probably deserves to die. I had high hopes for that game, and was just blown away at how shallow it was and how little forethought was put into the actual gameplay. PvP was ridiculous, there was no real balance in classes, and most of the group PvE was terrible. It looked decent, and I give them kudos for writing, voice acting, companion usage, and questlines, but those are all things that fit best in a single-player KoTOR game, which seems like what should've been made instead of SW:ToR.