I'm wondering if anyone has a link to some (obviously anonymized) tenure-track recommendation letters. I'm in the processing of putting together some applications and I've been curious to know what a good letter looks like versus a bad letter. There have been a few relevant questions asked here, such as this one, but it seems like for the most part I'm just seeing categorical philosophy about letters. I would be interested in reading a "great" letter and a "mediocre" letter, just to get a feel for what the difference looks like.
While I do have sample TT recommendation letters, I'm not at liberty to share them, even anonymized (sorry!). However, I've just re-read a few, and there are a couple commonalities I'll share below.
First, all made a strong case for the applicant having already impacted their field (for the record, all applicants completed a post-doc; none were straight out of graduate school). This is noteworthy, because recommendation letters for tenure seek to establish that person's real, significant contribution to the field. It is perhaps not surprising that the letter for a TT position has a similar flavour.
Second, all made a case that the work was likely to be funded. One of these had never previously received funding, the others were supported by post-doctoral fellowship or as some kind of co-PI on larger grant.
Third, all but one seemed to make an effort to highlight the 'departmental fit' in terms of research interest, both directly and complimentary.
I'm not sure how to answer your question about a 'great' letter vs a 'mediocre' one, but I did get the sense that the most compelling letters, in my opinion, had an air of 'obviousness'. What I mean by this is that the authors wrote as if 'obviously you must know this work' or 'undoubtedly you're familiar with...'. I can't be sure if that was actually true or whether it was a marketing ploy, but it seems to have worked.
Hope that helps, and good luck!