Legally, nothing (you should probably be asking the question the other way around). What DOES a podiatrist do that most orthopods don't do: foot and ankle surgery.
I have the utmost respect for orthopaedic surgeons. I've worked with quite a few this year. They have a lot on their plate WITHOUT worrying about foot and ankle surgery. You'll find that in this day and age, most of them specialize in one part of the body and that is pretty much all that they have time for.