When applying to faculty positions at teaching-focused institutions, recommendation letters that speak to a candidate's teaching ability are (presumably) very important. But often the person writing the teaching letter is a faculty member at a research-focused institution, where teaching experience of faculty applicants is not emphasized, and so they may not know how to write a good teaching letter (due to lack of experience evaluating such letters).
For example, Pete L. Clark said in this answer:
Many teaching letters are not worthy of more than a very brief reading. They simply do not separate out the candidate or provide any really incisive or useful information. If teaching letters are to be believed, then approximately 99.5% of all candidates are above average teachers. Why do I find myself skeptical of this?
What are the characteristics of a strong teaching letter? What can a letter writer do to emphasize a candidate's teaching abilities in a way that makes them stand out?
Of course it will depend on the candidate... but I'm interested in hearing from users who have been on search committees at teaching-focused institutions, regarding what kind of things they look for or appreciate in teaching letters.