This is just one factor of a letter, but I like when the letter writer gives examples of the applicant's qualities. It's one thing to say that an applicant works well with others, but if the writer goes beyond that and describes a situation where the applicant exemplified great teamwork or some other quality, that takes the LOR to the next level.
Also when the writer can give a sense of the relative strength of an applicant. Saying "Joe received an A in my class" is worthless. Saying "Out of the thousands of students I have taught in 25 years of teaching organic chemistry, Joe is among the top 10" is a lot more useful!