What can a 'general practice' doctor not do?

Depends. This may depend on the state in which the physician practices, and how rural an area s/he's in. Most wise physicians do not do procedures outside their realm of expertise -- like invasive surgery in a gp's case. I have always admired good gp's, as they're often the first doctor to see and diagnose an illness. They have to be very sharp in many areas to know what to do & when to refer.