I have played WOW since beta, and Aion since beta. That being said. I've played WOW into the ground and just don't want to play it anymore. Both games are well made and interesting. I find that Aion has a more interesting and well thought out world environment, while WOW seems to have a more easy to understand and ignore environment.
I don't really feel like we can judge them against each other. Of course people are going to think one is better than the other for whatever reason, but to judge them correctly, Aion needs at least a year to develop and flesh itself out.
Think back to the early days of WOW, was the endgame really all that big and exciting? Not really, there wasn't that much content. Aion will develop more endgame once there are actually people there to play it. And really, can anyone say they never had to grind anything in WOW? Ummm yeah, if you have rep with a faction you've grinded a few mobs in your day.
And there is no point in discussing graphics or customization differences. The games are two entirely different styles and made in two completely different manners. The furthest I would go is to voice an opinion of which style you prefer.
They are both solid games, and I think that we'll see Aion flourish and turn into a widespread game like WOW has if it can keep its player base for the time it needs to really turn into a home for players. Of course those of us that have played WOW for 4-5 years are more comfortable there, we've seen everything and always know what to do. After your first max level characters in Aion you'll feel that way about that game.
I think this discussion needs to be put on hold for about a year. Ask us once we've had time to fully explore the game, as well as given the game time to develop into its potential.