Priest is the strongest, most polyvalent, funniest, most challenging healing class in vanilla. And the only class worth playing overall in vanilla PVE. Also we can smite.
Druid is a PvP class and is only brought in raid to buff, innervate a priest, and not DE the loots.
Paladins are RDX spammer, which is ncie if you want to watch the champion's league while raiding.
Sham=CH bot. Totems are nice to have tho.