As for what is holding us back, there's ethical problems to consider. Certain diseases are obviously ones we'd want to get rid of, like Huntington's disease. However, would it be ethical to prevent autism? ADHD?
Ensuring that we don't wander into the realm of eugenics is really important.
If I was a parent I wouldn't want my kid to have cancer or something similar if I carried that gene. I would like for my kid not to have it so he or she can live long.