I haven't posted here for that long but I think that a more co-operative and cheerful style of discussion would strengthen the community, encourage people to post more, and ultimately strengthen the rationalist cause. In short, it would seem that adopting such more benevolent norms would be the rational thing to do...

I would agree, but "adopting benevolent norms" is probably not so easy to do in practice. Nasty people can be very subtle in their attempts to demean and put down other people. Probably a lot of the time they aren't even aware of it themselves. A concerted effort to improve the level of benevolence would be very costly in time and energy and would invite endless meta-debate.