The whole breeding aspect of fantasy worlds is fascinating.

As always, humans can breed with pretty much anything, populating the world with half-anything.

Really monster, are we that attractive? Won't you prefer to mate with that fat ogress with a big, round belly and a nice stub under her chin?

Biology apart, I think there may be some compatibilty problems between some of the races, but if we start putting logic there that player can't min-max his half-fairy half-dragon for maximum spellcasting prowess.