What can a letter writer say in a teaching letter to make a candidate stand out? 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.
I've served on many search committees in a mathematics department.  At our institution teaching and research are both important, and I wouldn't describe our institution as entirely "teaching focused."  

In my experience, most recommendation letters that address teaching don't show that the letter writer has actually observed the teaching of the job applicant and don't say much about what the applicant actually does in the classroom or how effective this was in promoting student learning.  I'm generally impressed by letters which describe direct observations of the candidate's teaching.  

e.g. "Jane had average student evaluations of 4.0/5.0, which is very good in my department." isn't very helpful (especially since I can read the teaching evaluations that the candidate submitted with the application.)    On the other hand, I'd like to see something like "As the calculus course coordinator, I met with John on a weekly basis and regularly observed him in class.  His lectures were well organized, and he regularly incorporated pair-and-share and other active learning strategies.  In interacting with students he consistently has a positive and encouraging attitude.  I have never had to deal with complaints from students in his sections.  John's students also performed well above the course average on common final exams."