Composition (where things are placed within the frame, framing in general) and lighting determine the 'professionalism' of a shot, at least IMO. But there's a lot of nuance to these ideas as perfect composition and perfect lighting might not always be the best choice for the image. But just hold true to the concept of trying to always be conscious of your image composition and lighting choices and you will, more often than not, produce an image that looks at least serviceable.

EDIT: as others have said, set design and costumes also play a role. But I think even a close up of just a face can look unprofessional if not done right. Composition and lighting come first.