There is one feature of Aion that WoW should absolutely steal: class channels.
In Aion, one of the automatic channels you join is dedicated to your class. All Clerics are in the same Cleric channel, all Templars in the Templar channel, etc. I find that these channels are invaluable.
Class channels provide a new or inexperienced player with immediate access to the wisdom of higher-level players of the same class. I've seen discussions on how to gem, advice on which path to take, and general all-around help.
I think WoW players would also greatly benefit from this. To have a place where you can ask questions, which is only populated by people of your class, would be very useful. Even the lurkers can benefit from seeing questions answered. As well, by restricting the channel to a single class, you avoid cross-class arguments and sniping.
In some ways, it's like the class forums, but more immediate. You can ask a question as soon as you have the problem, and get quick feedback.
The only concern is if there would be too many people in the channel for comfort. From my experience in Aion, I find that the class channels are mostly quiet, unless someone has a question.