The intelligent threads, definitely the humour, banter, droll one-liners. the fascinating depth of knowledge about all sorts of subjects.
Aviation and sailing tend to attract a more intelligent sort of person.
Some specialised motoring forums do also, then you get the ones like Barry Boys, who, like an accident, have a macabre fascination.
Oh, did i mention the way thread-drift is indulged and as a result, many fascinating tangents emerge?
All in all, the forum is entertaining (apart from R&N-I'm banned!)
light moderation is exemplified in the Aussie and asia sub forums, where they get a bit personal with the slagging-off, but it all comes right in the end.