Unlike almost any other cosmetic procedure, an excellent hair transplant requires an excellent team. The surgeon is only part of the equation. I am responsible for hairline design, donor harvesting and closure, and recipient site creation (where the grafts go into). My team is responsible for graft dissection and placement. Having an excellent team is mandatory.
Almost any surgeon/physician can do hair restoration, but as the other physician has answered very few do it well. Looking at one's before and after results can be a testament to one's skills.
Knowing as much about safety regarding when to transplant and when not to is very important. I believe that good judgment must involve both understanding the medical aspects of hair loss (types of hair loss that you can't operate on, medical management, etc.) as well as surgical expertise (artistry and technical skills).
Finally, ethics is key. Too many clinics have hard driving salesmen to push you to do a case that the surgeon might otherwise think is dangerous or inappropriate. Ethical judgment on when not to operate is very important. hope that helps.
p.s.: here is a short video in which i talk about if any physician is trying to get into the field, what he or she should be thinking of regarding all of the above points.