Writing letters is an art. A lot of letters are the same stuff. Here are some of the points that people seem to make in letters:
1) How smart you are - scores, how you got some obscure pimp question correct, etc
2) How likeable you are - that's cute but they need to "show" this with an example and it should be a good one
3) Research - some writers just know how to make this sound good. They can talk about character related points and that make it a much more rounded discussion.
4) They have to tell the reader why they should like you and I mean really like you. it's got to go beyond scores and objective factors. the subjective factors have to be believable. It's really easy to tell when a letter writer puts zero effort into a letter on this part. This is the part that is the big sell in letter. You need the other parts but this is where the magic happens.
If you get 1, 2, and 3, you wil have a strong letter based on how good the writing is. If you get 1-4 written well, the letter has a chance to make a real mark.