Druids were once one of the least played classes, but I see them all over the place now. I also see quite a few death knights, of course, and more shamans than I would expect (given that I play Alliance). I've also never been able to swing a stick on my realm without hitting a paladin. Warlocks are probably the next rarest class I see, after rogues. I'm aware that this varies realm-to-realm, though - what's the least popular class on your realm, according to your experience?