I have one 80 on Horde side and a few 80s on Ally side, and it seems that the more rare classes on Ally side are Priest, Shaman, Warlock and Rogue. Hunter and Death Knight kind of in the middle, whereas the most frequent classes are Mage, Druid, Warrior and of course Paladin.
As far as Horde side are concerned there are more priests than Ally side (mostly because of blood elves) and more rogues as well (cause undead have always been the coolest looking race ever). Also there are a bit less druids on Horde side as well because there are a lot of female night elf druids on Ally side.
Tanks are definitely in the highest demand, with healers coming on 2nd place. If you want to really be useful but still not have a common spec you should go Bear Tank or something like Resto Shaman.