I find that there are some characters that showed up a few times, and others that only showed up once or twice. Some of them were great and I'm surprised they didn't show up more often. I'm sure contractual obligations have something to do with it, too, but still. (They used 2 different actors for Lloyd Braun, and while I think he ok, he didn't make me laugh THAT much). There are others that showed up less often that I thought could have been way better semi-regulars. Here's my Top 5 list of "Characters I wish showed up more often".
1) Mr. Bookman
2) Jackie Chiles (even though he was on 4 or 5 episodes, he could have been a regular as far as I'm concerned)
3) Elaine's father
5) Izzy Mandelbaum (R.I.P. Lloyd Bridges)