Gnomes will be uncommon.
Priests on the Alliance side will be minimal, witch my experience in the betas I was probably one of 25 priests I saw in the whole time.
Shadow priests are much more common however.
Shaman are very common as well as paladins, and I'm sure the Druid/Warrior classes will climb since the last patch.
Some of the more experience players in the betas were leaning away from making a Shaman mainly because what with all the peopel trolling and whining about how great the class is, we might end up with a nerf or two to us.
The honest truth about shaman is that they have a bit of skill in every area, sure they are good at everything, but they aren't great at ANYTHING. Each class has a unqie thing they are masters of, Priest heal, Warrior fight, and Shaman just do everything moderately well. This is not a crime, nor should we be the victim of some newbie crying about trying to take on a Shaman in the wrong way his class was built for and dying (E.g. melee as a mage, pssssh, you're wearing cloth and have tons of distance abilities, take advantage of it.).
Thus I say the shaman class will be overly full, but some peopel will be anxious to go put their soul into a character that will be crap par next patch.
My 2 pesos.
Kahrain