What is the other half?
Which (semiseriously) is the problem with half races. Since half human, half-X seems to be the assumption, I keep thinking that it would make more sense for the half races to be human subraces (maybe with a less restrictive name like "-blooded" ["multi-blooded" could be the default human]). It also might lead to more interesting traits and feats for poor humans, who tend to get the dull ones.
That is because humans are dull.