I think it is difficult to combine self moderation with formal moderation. Either you have formal moderation, where the opinion of the mods is all that matters, which has the downside that the mods may be a bit stricter (or a lot less strict) than most of the people using the group would like. Or you have a self moderated group, where it's up to the group as a whole to express disapproval. And, of course, with a self moderated group, you always get some prat whose enjoyment is annoying people, and there's no way of stopping their posting ... which is why newsgroup-reading software comes with a variety of 'kill' mechanisms to block posts or threads or subjects which you feel do not add to your enjoyment of the group.
I think combining the two would only have a chance of working if most of the posters and the mods had the same expectations of the what the group is about. The fact that the shit heap is so large shows that this probably isn't the case in ukc.
I know some people are worried whether the amount of silly stuff will put off new posters. I wouldn't have thought so. When I joined, it was for the caving content, and I didn't even look at 'idle chat'. It wasn't until I got to know some of the on-line personalities that I started getting involved in 'idle chat'. I would, however, have been put off by looking at the range of forums and seeing a section labelled 'the shit heap'.
Personally, I enjoy 'idle chat' - I use it to wind down after a boring day at work. And although it's silly, we have very few offensive posts - I can remember only one such poster, and he was quickly banned. I suppose there is an argument that we shouldn't let serious threads degenerate into idle chat. Is there a way a part of a thread can be moved, so that once the serious thread starts to degenerate, it can be moved across to idle chat, without having to move the informative stuff as well?