I think the game threads are largely a reflection of the site itself. There's a lot of negativity here. It's frustrating, but I'm not sure if it's any different than going to a Celtics game when the team is playing poorly.
The answer still isn't to segregate posters onto separate but equal "negative" and "positive" game threads, though. Dividing posters by viewpoint goes against everything Jeff wants this site to be.
At the same time, we do have the option of restricting members' access to the game thread forums if they, among other reasons, are "systematically negating the ability of others to enjoy this site". It sounds like this is the type of behavior people are objecting to.
What I'd say is, if there are individual posters who are disrupting the game threads, report some of their posts, so the staff can take a look. If appropriate, we can look at restricting access as an option.
I would note, however, that this problem is at least in part self-created, albeit collectively self-created. It seems as though many members have left the game threads because they're too "negative". However, for every person who leaves, that's one less "positive" post. From my perspective, folks can't really complain about the proportion of negative-to-positive posts, if they're abandoning ship (which effectively makes the negative-to-positive posts even worse).
In an ideal world, all of the folks who want to cheer on the team would band together, and continue posting their thoughts, in an effort to drown out the "negative" ones. I know this isn't necessarily a realistic goal, but if all the positive / optimistic posters leave, they shouldn't be surprised when all there are is negative posts left.