The sound effects in Aion bother me. There's nothing necessarily wrong with them. They sound like the kind of effects you'd hear in any JRPG. And I'm a huge fan of the Final Fantasy series which has similarly video game-y sounds. But, for some reason I can't quite explain, it bothers me in an MMO.
The jarring sound the guards make when you alert them in Metal Gear Solid doesn't bother me. But the metallic beep-beep sound that occurs when you draw aggro in Aion irritates me to no end.
I guess I'd rather hear the bear growl or the monster shout or something a little more "realistic" in an MMO. I don't want a constant reminder that the virtual fantasyland I'm immersing myself in is just make believe. But that's exactly what the video game sounds do for me.