There was some general frustration with the forum. The Transparent Mod Log thread has a few of his more candid responses (starting around
here). His frustrations, among other things, seem to have led to a few of the newer "veteran" threads started by Manton and others.
Beyond that I think he just grew tired. The majority of his posts were snark towards the end. Really at a certain point you outgrow this place. At least that seems to be the case for many. It's not hard to understand that he was on a whole other level from most of us. At a certain juncture, no matter how much you love kids, the buffoonery of toddlers loses its charm.