I haven't been around here long, (and I've only been growing a few years, so I still consider myself somewhat inexperienced) but I do have some experience with forums. I run a fairly large one. I found that as the forum grew and more people noticed it, the quality of discussion definitely declined. It was a very amicable forum when there were just a couple hundred members, but it seems that as it grew, the older, more friendly members grew tired of the newer members coming on and being negative. Kind of a snowball effect.
I think if the people that are positive stick around, and continue with good advice and encouragement, a balance will eventually be struck. Everything goes through phases, and perhaps a more negative vibe on RIU is just a phase? I hope so, because although I've seen a lot of nastiness in my short time here, I've also found some great info and some really great people. To those who are positive: please stick around for the sake of those of us who appreciate your advice and experience!