I think (albeit on the basis of limited evidence) that what's helpful for good conversations is a low value on humour rather than a negative value on humour. The fora I've seen with the best discussion don't generally regard humour as bad; they just regard it as generally not good enough to redeem an otherwise unhelpful comment. Exceptionally good humour, or humour produced incidentally while saying something that would have been valuable even without the humour, is just fine on (for instance) Less Wrong or Hacker News -- but comments whose only point is a feeble witticism are liable to get downvoted into oblivion.