It is generally acknowledged that one should try to get recommendation letters (for grad school, jobs, etc.) from well-known experienced members of your field, if at all possible (for e.g. see some of the answers to this related question: Do letters of recommendation typically include a biography of the writer?). Of course, sometimes this is not possible.
I am a first-year postdoc (in mathematics, but perhaps this is not relevant), and a masters student in the graduate-level class I am currently teaching approached me to write a letter for them when they apply for PhD programs. I am a natural person to write a letter for them since they are interested in the field that I teach (topology) and I'm teaching one of the three courses they are taking (since application deadlines are quite soon, they won't have taken any other courses here before applying). Of course, I am also a poor choice (which I mentioned to them) as a relatively unknown person with little experience.
While it makes sense to me that recommendation letters from senior research-focused faculty are worth more (since they have greater experience interacting with graduate/soon-to-be-graduate students), there are a fair number of students from relatively obscure 4-year universities who apply to graduate school. They might not have had any access to senior research-focused faculty.
What are some things I should keep in mind when writing a recommendation letter for graduate admissions as a new entrant to my field, or perhaps as an instructor at a relatively obscure primarily teaching-focused school?
Statements like 'they are in the top 7 of all graduate students I've ever taught' carry little weight, since I've only ever taught 7! For what it's worth, the student in question is doing quite well, and my goal is to write a well-deserved relatively glowing letter; I would like to make sure, as much as I can, that my letter is not ignored.