I am not sure why this post on the dozen sentences you need in your letter of interest has gained so much traction, but it has. It does not seem to include any information that immediately pops out as useful or novel. In fact, I think it misses lots of key points and information. More importantly, it is focused too much on being a junior scholar and a graduate student. This is a bad move for this modern job market. It is almost necessary for an applicant to be finished with their PhD and have a number of publications to succeed. Of course there are exceptions, but these are rare and typically come from elite schools. This leads to my first point: 
Keep in mind all these points are my perspective (which is often wrong). Also keep in mind I speak from the perspective of one that has been at research schools. Obviously a teaching college will be looking for different pieces of information, or at least, a different priority on each piece of information.
1) You are no longer a student, stop acting like one. Don’t tell us what classes you took and what your dissertation was about. Move beyond this. Tell us what classes you are prepared to teach. What methods training you had and the lengths you went to obtain this degree of preparation. Don’t tell us about your dissertation or your field work, but tell us about your research, your work and your contribution to the field.
2) When you speak about your work, focus on being brief but also have some clarity about what the contribution of your work is and where you expect to send it (or where it was already sent). We want to see the level of publication you are shooting for (and this does not mean that every ABD should be submitting to APSR in the hopes of getting a job because you have high aspirations, these goals have to be realistic).
3) Next, you should also tell us about the direction of your dissertation (try not to mention the D word though). Do you have a monograph and where to do you intend to send it? Is there already interest, a contract, do you have a proposal? Are you going to work on a grant proposal to expand your current work or immediately jump to a new research program? What is your post dissertation project and how will you tackle this project? We want to know where you are going as much as where you have been.
4) You should start the letter with the job you are applying for and also why you would like this job. Is there a certain reason this location is important for you? Is there a research cluster there that you fit into? At all stages, the candidate must put some research into the school to know something about their characteristics and style. This should come out in the letter, but this also does not mean you can’t have a shell letter ready to go and modify it by school. The idea that some advocate that each letter should be wholly unique is absurd given the amount of jobs many people on the market now have to apply for.
5) Tell us what you want to teach and why. Also, if the job ad lists a certain teaching request, tell us why you can cover these classes. This is key for many jobs since not every school is looking for the best researcher (although many are); they are often looking for the best fit given their needs.
6) Hint at your personality. I like to see some personality, some humor in the work I read. This can be done when you mention your background, your field research, possibly your education. The key is we are not hiring a drone, but a colleague, someone we want to spend time with for possibly the rest of our careers.
7) Finally, tell us who wrote your letters of recommendation. I am going to say something that might be shocking, but we don’t read everything. My tactic is typically to read the CV, if interested I will go forward and read the letter of interest and then if still interested I will read the letters of recommendation and papers. Having the letter of rec writers listed in the letter of interest is helpful in that a certain name might flag my interest. Do you have a writer who was not at your PhD granting institution? This shows you are connected to the field as a whole. There also the issue of the missing letter, who did not write a letter who probably should have given your school and research interest, letter writers are as much signals as sources of evidence about capability.
8) Beyond the letter you should have a decent web page. This must include an updated CV and links to your publications and conference papers. I am constantly amazed at the candidates who don’t have this. Sometimes you don’t make the short list but are given a reconsideration during the short list meeting. At this point we might not have the required background in front of us, so a good webpage is key. The other possibility is the search committee might select a short list of names and send those out the to the department without the files and CVs attached, so it is your interest, if you want to build a case of support, to have your work easily accessible.
So what do you like to see in letters? What advice have you received?
