An agent recently offered a friend of mine representation. I heard about this first thing when my friend pinged out critique group with a frantic message about what he should ask before inking the deal.
It was eye-opening for me, since I've got material out and if I got the call today I would be wondering the same thing. Luckily, shortly thereafter a couple of agent blogs made excellent posts on the subject.
First Jenny Rappaport gave a thorough list, and not long after Nathan Bransford invited a guest blogger, Ginger Clark, to chime in on what you should cover. There's a lot of overlap between the two, and the bottom line seems to be about setting expectations (like any good relationship!)
I still plan to sit down and write up my personal list distilled out of these sources, but these give a great starting place for that moment of truth.