The same thing any other LOR would include. In what capacity they know you, your strengths and how you shine maybe even your weaknesses and how they've seen you improve upon them and mature. I actually worked with a DDS (dentist)who wrote one of my LORs and at interviews I got several comments about the strength of the letter, especially specific anecdotes and examples backing up the things she was saying about me. Also an endorsement about how they can envision you as a colleague one day doesn't hurt. It really doesn't matter that the letter comes from an MD so much as what that recommender has to say.
Hope it helps