Some of the servers are jam-packed, while others have plenty of room to grow (no labels). NCsoft has temporarily locked new character creation on the higher population servers until the lower population servers catch up. To maintain faction balance they are also keeping track of Asmodian and Elyos populations. If one side becomes too dominant, new characters of that faction are disabled until the other side can catch up.
Many people did not like this and understandbly so, but I think it was a fantastic decision for the long run health of Aion. Many people probably remember all to well how unbalanced the WAR servers were on release. The unbalanced server population in WAR greatly contributed to the decline of the game. It is something that, a year later, they have still not recovered from.