Aion and Wow are pretty different in focus. The two games are actually quite a bit different. Aion succeeds in the area of RvR combat which WoW does not have much of. WoW's pve in particular events, raids, factions are simply the best still thou. WoW will continue to dominate these areas. In regards to pvp, Wow is focused around small group (arena / BG) vs Aion's large / medium group RvR pvp both styles can be fun. I do feel that Aion will be a success most due to this different focus on RvR.
My own issue with Aion is that getting to the pvp in Aion feels too much like a chore. Being forced to go through Aion's questing and pve felt almost like grinding. The 100 possible deliveries for funds is an example. Since the majority of WoW players in NA / EU are pve players, I do not really feel that WoW will loose alot of these pve primary customers. However, Aion will hammer Warhammer into oblivion as well as pull some more pvp oriented customers from WoW and other MMOs.
