AAMC had a document called Guidelines for Writing a Letter of Evaluation for a Medical School Applicant but it isn't on the website right now. You might look for it later as it could be down while the website is being reconfigured. Maybe someone has saved the pdf and can send it to you.
The letter should include, IMHO, only things the writer has observed. The writer should not restate items in the experience or personal statement if they do not know of these events first hand. The exception to this rule would be the committee letter that summarizes information provided in letters submitted to the committee. So someone shouldn't write that you served 800 children in Peru during a mission trip unless they were on the trip with you or are writing a committee letter.
The letter should give examples and address academic achievement and classroom (or laboratory) engagement but also include a paragraph on demeanor, ability to cooperate and work collaboratively, communication skills, response to hardships (e.g. applicant was not discouraged by a setback but determined what went wrong and/or how to improve and set things right).
A good letter doesn't go on and on about how great the writer is and how prolific the lab is. It makes statements about the candidate and backs up those statements with examples from the writer's personal experience with the candidate.