In the words of Linda Richmond, the dean's letter is neither a letter nor written by the dean. Discuss amongst yourselves.
Seriously, the dean's letter usually isn't written by the dean him/herself. They have delegates who perform that task, and it is up to you to provide these individuals with all of the information they need to paint an flattering picture of you. The letter usually begins by detailing your undergraduate career and accomplishments then moves onto your residency goals, extracurricular activities, awards and honors, class standing, and any other pertinent details of your time in medical school. At the same time, they try to convey what sort of person you are to the residency program.
Following this are narratives from each of your third year clerkships - brief snippets of quotes from attendings who can comment on your clinical abilities.
Finally, the letter concludes with a few paragraphs summarizing all of your achievements and the final recommendation of the dean. Many residency directors just skip to the last line of the dean's letter which is coded to say what kind of student you really are: e.g. "my highest recommendation without reservation" = one of our best students, or "this student will do a capable job in your residency" = at the bottom of the class, etc. It's not really a letter of recommendation more than a way to convey your class rank, honors, and how the school feels about you to the programs.
AV