So much suffering is caused by genes, from physical to mental illnesses. It's hard to imagine what would be wrong with a world where no one got cancer, became schizophrenic, or perhaps, even suffered from depression. The big danger I guess is that we end up deciding there is only one (or perhaps a couple) valid human "types", and eccentric perspectives that create value (in any sense) are lost, not through extermination, but because they just are never created to begin with.
The gradual improvement of the gene pool will likely happen anyway, as parents choose to use new technologies to improve their future children in various ways.