The problem with this is that I don't want to have to figure out where the line between discussing concerns and being labeled "anti-technology" or a "luddite". I don't think I'd likely be miscatergorized, but I could imagine it being very frustrating, and create a wet-woel effect on conversations that could be useful.

I think encouraging each other's optimistic perspective, through voting and interaction, will do a better job than prohibiting the alternative. Make the optimistic threads exciting to be a part of, and let the contrast of the pessimism be do the convincing for you.