Just remember that in academia, all bets are off. Sure, rules and guidelines apply even here, but many professors don't know them--or if they do, ignore them. Consequently, references from university profs often go far beyond the typical name, rank, and serial number, for better or for worse.
One additional caution: Beware the letter than damns you with faint praise. As an undergraduate, I got badly sick one semester and received a very bad grade in a physics class. My professor in that class despised me. I took the course again as an independent study and got an A. Triumphantly, I asked that professor for a letter of recommendation as a way of addressing that blemish on my transcript.
Bad idea. He wrote a one-sentence recommendation saying that he was sure that "I would make a fine graduate student," or something. I got into grad school, he's probably dead by now, and my physics career has come and gone. But I admit it: I'm still a little bitter.