So what makes a good letter of recommendation? Well i'll try to pass down some info from what i hear from my professors. First off, letters of rec support or provide evidence that you did something. For instance, if you did research, and your research professor writes you a letter, more power to you. Secondly, a strong letter should encompass many points that aren't just limited to GPA/MCAT, why you are affilliated with this writer, etc. The more personal letter is, the better.
So thats what a good letter should be...give or take a few other things which i can't think of at this moment. Now consider another aspec; you letters of rec as a whole. Obviously schools want 3-5+ letters of rec. It behooves us to get letters of rec that consistantly has the same tone. (ie: This person is GREAT!). I don't mean the standard..."yea this person should get into medical school", but reinterate something unique and very positive. Pretty much, if 5 different people from different fields say the same thing about you in a great way, then that at least brings up a flag to the reader...."maybe we should check this person out."
However, despite the power of an extraordinary letter of rec, the problem for many is getting to the point in the application process where the adcom can see your letters. This is the case with the University of California School's of Medicine. These schools screen for GPA/MCAT's first, and then may or may not ask you to send in letters of recommendation.
Sadly GPA/MCAT are at the top in regards to what gets the ball rolling. The personal statement is also very important too, since it can help you (if supported by something) describe discrepancies found on your GPA/MCAT. However, schools need a quantitative way to measure one's ability to succeed in medical school, and that would be grades. If one has low grades/low MCAT, and no external reason for this, while having tons of extracurriculars, then they're in trouble. It may seem that this person put in more time doing the EC's (albeit more fun) rather than studying in school. At the end of the day EC's serve as icing on the cake. It shows that people who do well in school have a life outside of school. (ie: a human) Letters of rec at the very least validate what you did in school and your EC's, while providing the adcom's with another perspective of you as a prospective doctor.
Now for your MCAT, i'm not sure if the average has changed since i last checked (2-3 years ago), but the national average MCAT score for people getting into med school was around a 28. I know that the UC's are around 32. WHat has been said time and time again is that one not only has to get a high MCAT score, but also a good even distribution of scores per section. So 10,10,10 is good. But 10,10,5 is BAD. I think the rule of thumb is anything less than 8 isn't competative, especially for science majors. Everyone should be shooting for at least 9's across all sections, if not more.