Blizzard has implimented a new system to help control character populations on servers. Eyonix described it to everyone in the post below.

We've implemented a new system that will disable character creation when a realm has a queue to better control realm populations. This system will disable character creation for accounts without characters on an individual realm when it has a queue. This will limit population growth on congested realms during peak hours and assist us in our efforts to provide a better overall gameplay experience for our players, especially those on high population realms.

Please note that players with existing characters on a queued realm will still be able to create characters, and during off hours when there is not a queue all players may create characters as normal.