Their mucus turns you into one, they have psionic powers that aren't too shabby, and in several cosmologies they take over/nearly take over. They royally screw FR, for instance.
Humans aren't just silly, they are ludicrous. Free skill points, bonus feat, favored class: any, and no stat penalties. Sure, they're one of the most numerous species in any world, but that's just simply silly for such a superrace.