Not that I'm any kind of expert but OWAL and Mark have it spot on couldn't have said it better myself.

My my theory on humans having so many negative genetic traits currently is due to the very short amount of time (relative to other genus) that we have been around. Add in the fact that our species got lucky with the brain traits we have allowing us to master the food chain and you can see how our negative defects never really got a chance to be selected out. Humans had it very easy historically compared to every other species out there. The brain really got us into a position where we had abundance and therefore we do not have as direct of a Darwinian pull against negative traits. Even before civilization we had much better lives then most other species.