Plus it really depends on what you want to play. If you want to play a Paladin, you must play Alliance. If you want to play a Shaman, you must play Horde. And there are other factors regarding classes that will shift people to a particular faction.
However, if it does become wildly unbalanced, I wouldnt be surprised to see a second pvp server created to try and address that as soon as possible. After all, having this name saving thing happening means they have a pretty good idea now who is going where.
They also said:
We’re considering integrating a character transfer service once players have reached higher levels. This was available in the original WoW.
That being so, if things did get bad, players would at least have the option to swap over to another server.