Norm Geras is a liberal who cannot fathom why the left is hostile to the concept. Money quote:
No, it doesn't make you a rightwing anti-egalitarian. It's one obvious basis for a philosophy of universal human rights - the idea that we share a common human nature, and by virtue of it common needs, common capacities and fundamental interests. These interests ought to be protected. Granted, the fact that a common human nature provides the intellectual basis for a justification of universal rights does not by itself show that there is such a common human nature. But there is one - as any open-eyed consultation of historical, anthropological and cultural evidence will vouchsafe. We can argue about what traits it includes and what traits it doesn't - a really important area of argument - but the denial of there being any human nature at all is wasted breath.