Aion never had the potential to be a WoW killer, it didn't have the budget for it to even be an outside possibility, even if it had genius game designers.
Aion is basically "RF Online rides again".
I've seen the same thing as everyone else pretty much. A lot of people went to the game to see if it would be new and incredible, it wasn't, and was actually pretty pathetic on some very basic levels, and then pretty much went back to their other established MMORPGs when thei initial subscriptions ran down.
It reminds me of RF Online, because that was supposed to be the big "Asian WoW Killer" that featured concepts like a class that could pilot giant robots. Similar to how Aion had the whole gimmick of totally 3 dimensional play (ie everyone can fly). In the end it went from being discussed in the gaming community, with everyone talking about trying it (or knowing someone who was going to), to abandonment, silence, and eventual obscurity.
Truthfully I do not think WoW will get a run for it's money until "Knights Of The Old Republic" comes out. Barring the possibility of some surprise uber-game that just happens to come rushing in from the shadows (like say Star Trek Online becoming the holy grail of gaming for whatever reason). I say KoToR because it apparently has a huge budget, and a legendary design team like Blizzard behind it (Bioware) who are apparently being given enough of a free hand to do it right, and take as long as they need.
It's always possible that like Age Of Conan KoToR will not deliver half of what it's promised, and join the fail train of "3 month wonders" (ie free first month, plus the 2 month game card many gamers will buy at the same time they get the basic software). I think right now KoToR's biggest challenge is simply going to be having enough content at launch to keep people occupied for months on end. Basically if your getting into a mindless grind/farming rotation (even an endgame grind) within 90 days chances are the game isn't going to be able to seriously compete with WoW.
