I've only played a little bit since it went free to play (busy with Tera and The Secret World), but I don't think Aion is pay to win. I didn't see much in the item shop that suggested it would affect your play much. Mostly just vanity items and some 30 day mounts, as far as I could tell.
The biggest complaint against Aion I remember while it was still pay to play was the level grind (very typical Korean level grind at the time it was released). They've since made leveling alot easier and faster since, so that shouldn't be nearly as much of an issue now.
Another thing that could be good or bad, depending on the person - the combat feels very WoWish to me. I think most people who played WoW would find the combat familar.
The biggest issue of it for me when the game was released, was the forced pvp. I'm not that big into pvp myself. Which makes me wonder why I played it in the first place, since it was billed as a pvp game.
But, there were pvp zones (the abyss), and non pvp zones (pretty much everywhere else), so I thought I was safe. Besides not caring for pvp, I'm also a slow leveler, so I fall behind the curve quite quickly.
But, the biggest problem with pvp was Rifts. You'd have a rift open up in a non pvp zone, and membes of the opposing faction would be able to pour in and attack you in "safe" zones. The number of people going through a rift was limited, but that number was failry large. And, they could bring kiosks with them, which would allow them to revive in zone, instead of having to go back through the rift every time they died.
Which meant that I was constantly getting killed when I tried to pve, and leveling slowed down to non existant. The worst parts, was when they weren't killing me after I had already engaged a mob, they were wearing pvp gear far better than mine, and were usually 10+ levels higher than me, so it wasn't a pvp battle, just a slaughter.
I know right before I had dropped my subscription, they had changed rifts to only work every other week, so you could actually pve in peace every other week. I dont know how it works now.
the thing I liked best about Aion was it's character creation system. I personally haven't played a game with more customization than Aion. I wish more games would give that kind of flexibility when creating characters.