They throw gender into the mix as well -- turns out that while the classes have generally the same percentage of players (not surprising, given that gameplay dictates the classes should be fairly balanced), things start to break up when you add gender to the mix. Priests and Warriors seem to have the biggest separation: according to their data (obtained via the profiles on their site), most Priests are played by females, and most Warriors are played by men. Paladins as well tend to be male, though not as much as Warriors, and Druids tend to be female, though not as much as Priests. Women also tend to prefer the elven races (Blood and Night), while guys apparently prefer Orcs and Dwarves (which helps my -- sexist, I admit -- theory from way back on the WoW Insider Show that the Dwarven starting area appeals to guys more than women).
The Bartle breakdown is interesting, too -- Killers prefer Rogues (duh), Warriors tend to be Achievers, and Hunters have the slight Explorer edge, but in general, the classes have a fairly even distribution across the board. All of the different roles can be filled by all the classes, which speaks to the way Blizzard has built the classes -- you can really solo, PvP, or group up with any of them. WAR's differences were distinct, but in WoW, Blizzard has done their best to make it so that whatever Bartle type you are, you can log in with any class and do what you want. gamerDNA promises more research here (including a Horde and Alliance breakdown), and we can't wait to see it.