Keen has two blog posts up discussing the similarities and differences between Aion and World of Warcraft and Warhammer Online. The first thing you should bear in mind is that he has not played Aion all the way through to the end game, even though he has "finished" both other MMOs. The second thing of note is that these points of comparison were requested by his readers.Even if one has not played Aion's end game, one can still make a valid comparison of certain aspects of it -- one just can't compare everything. There are a couple points where Keen defers to others who have more experience with the Korean servers and Chinese beta.
Even with the level 20 North American beta cap (weekend #3), you'll get a pretty strong sense of where Aion is going because that level is 40% of the way to cap. Head on over to Keen's two blog posts (Aion vs. WoW and Aion vs. WAR) and see for yourself. The comments are loaded with good information too.