I've seen pretty much every combo often enough to not be impressed by it anymore, or do a double take. Looking at my Fem Pandaren Warrior is pleasing and new enough to make me consiously watch though.
worgen warlock/rouge/death knight.
dwarf warrior (thank you J.R Tolken).
NElf female has to be the most underrated race/gender in the game. They look ugly until you get them in higher-level gear, though.
Same goes for Draenei Males. They look horrible until they have some real shoulderpads on.