Demo locks and resto shamans are in pretty high demand (alliance). But tbh aslong you aren't a plate dps you shouldn't have any problems finding groups for raids and such.
PvE: DK, Paladin and Druid tanks. I also second on esjon's suggestion of warlocks - mainly for their buffs and convenience - and shamans on alliance. Horde would probably need more holy palas.
PvP: You'd be surprised that we need more plate dps, especially ret paladin. This is because of the low population making Warsong Gulch far more prevalent. And "frontliners", or plate dps who take the lead, are ALWAYS NEEDED to overpower the enemy team. Aside from this, resto shaman and druid. We already have a surplus of priests.