What would you consider...

<p>an excellent letter of recommendation?
What qualities would it have? (specifically)</p>

<p>There would be details specific enough that they could only be describing that one student and no one else. Also, this website provides a great overview of the characteristics of an excellent letter (even for people who aren’t applying to MIT):</p>

<p><a href=“http://www.mitadmissions.org/topics/schools/writing_evaluations/index.shtml[/url]”>http://www.mitadmissions.org/topics/schools/writing_evaluations/index.shtml&lt;/a&gt;&lt;/p&gt;