I run about 25 Yahoo! groups, and one large, busy Mailman list. I have also run lists with Majordomo, Onelist and eGroups in the past. I also belong to a number of mailing lists that I don't own or moderate.
From all this, it seems to me that there are two basic philosophies about e-mail lists (often mistakenly called listservs, after one of the early software packages to support this.) Some people regard lists as a kind of virtual water cooler ... a place to hang out and talk about whatever comes to mind, often but not always related to whatever the participants have in common. Thus a list for taxidermists may have discussions on where to get "authentic" pizza, which political candidates can most easily be likened to Hitler, and who has a video of the last episode of Lost to lend.
The other view is that an e-mail list is like a spigot. I join the taxidermists list because I want to talk and hear about taxidermy. If I want to talk about other things, I can join the pizza list, or the Hitlerian candidate list, or the Lost list. When I turn on my hot water, I don't want radiator fluid to come out, even if it's really awesome radiator fluid.
Invariably, there will be people frustrated with whichever approach the list moderators take. There's no walking the line. Some moderators have created two lists ... an on-topic list and a chat list. In my experience, this is about as effective as a "Yield" sign on the highway.
I'm firmly in the spigot camp, but I've learned to let the lists I moderate seek their own center. I get occasional complaints from one side or the other, but I can live with that.
I have a bunch of other pet peeves about e-mail lists, but I'll save those for a future posting.