I have posted this before, but:
Any good LOR will do several things:
1) Attest to your intelligence, intellectual curiosity, academic strengths, etc
2) Attest to your ability to work as a member of a team (What kind of a person you are to work with, friendly, helpful, rude, always late)
3) Attest to your motivation and interest in the field you are applying to (People in your field of interest, obviously, know what the field is like and are best equipped to evaluate this, but others may be able to contribute something useful. A surgeon, for example, may talk about someone's interest in pathology and how they were a great help in learning about these issues in regards to your patients).
4) Talk about what kind of a resident you would be (likelihood of completing assigned tasks, ability to work under pressure, working long hours, working under supervision, accepting and encorporating criticism)
5) Talk about what you are like as a person (People like to work with people they like. No one likes a brown nose, no one likes a snob). Preferably, this will include personal details that have to be gained by spending time together and facing challenges together.
6) Have some insight into your future career plans and how your qualifications stack up with these ideas (Do you like research, plan on academics, are you a budding teacher?)
A great LOR will do all of this, and probably convey in it that this is the kind of person the letter writer wants to see succeed, and would like to work with in the future as a potential colleague. The best compliments doctors can make are when they refer to someone as one that they trust with their difficult decisions and patients.
Obviously, at the early stage of your career (not yet in residency) not all of this will be included. But people can often see things in students that points the way towards a bright future. And they will definitely accentuate your high points and positive characteristics and minimize or ignore your failings, unless of course you asked the wrong person to write a letter.
It is a good idea to have at least one letter from a pathologist. But you certainly don't need all of them. Mine were, but that was where most of my experience was, and where I would get the best letters. If you have a letter from another field which talks about how intelligent, friendly, hard working, etc you are, this is going to be a good letter for any field.
