Remember though, it's the quality of the letter that counts (not the title of the author).
Agree with this. MD is the best choice, all things being equal, but if they can't write you a decent one, volunteer coordinators are fine.
So how do you know if the letter is good? Well, if you're close with the author of the letter, perhaps he/she will give you a copy of it or let you read it. I wouldn't ask for it directly, as that would be too bold.
Don't expect a copy of the letter. Even if you are close to the author. Many medical schools will not accept letters of recommendation unless you waive the right to see them. Even those that don't have this policy prefer letters you haven't read. This lets the letter writer send in honest impressions. Many physicians have been asked to write letters and are familiar with this policy so they might not send you a copy. Asking for one is just rude.
Perhaps the author will allow you to provide inputs, like a bulleted list of all the great things you did.
Always do this with letters of rec, though I wouldn't call them inputs and I'd avoid bullet points (which look more demanding). After a letter writer agrees, I usually tell them I'll give them a letter with the address and instructions. I then write them a letter thanking them for their help and give the address. I also include something along the lines of:
"This school requires three Letters of Recommendation not to exceed 1-2 pages. I am sending in one from ___ that outlines my research experience and one from ____ that talks about the shadowing I've done. What I'm hoping that your Letter will touch on is how I interact with patients and any passion I've demonstrated for medicine, etc. etc."
If the author is completely secretive about it and wont allow any input, that might be a red flag.
Keep in mind that many (most?) physicians are control freaks by nature. They might be open to your asking what you'd like the letter to focus on, but don't be surprised if they don't want you to direct it or if they don't give you a copy.
The way you determine if a letter writer is going to write you a good letter is by asking directly, "Based on our time together, would you be able to write me a
strong letter of recommendation?" If the potential writer says, "Uh... maybe..." then walk away. If they say, "sure", you're gold. MDs and volunteer coordinators understand what you're after.
Best of luck to those in the upcoming cycle...
Good luck to everyone that continues to wait!!! 
Yeah, I definitely take it back!
