Lock/move to 'Dead Threads' (Should make one of those).
"Dead Thread" sections create unnecessary work for the mods, in my opinion. It actually makes things less organized because people looking for specific old threads would have to sift through a bunch of unsorted ones rather than the ones in a specific area (e.g. GameMaker Help). If a mod truly thinks that a thread is over and that there should be no posts made in it, it can be locked, but most threads on this forum are never really dead because something can always be added to the discussion. That "request" thread which turned into a flamewar was an outlier on this forum in that it actually deserved a lock; the GMG seems to be civil most of the time. I don't think there's a need to put more restrictions/rules in place.
I also don't agree with posts being deleted unless they are clearly spam.
Another weird aspect of this board is that most of the active members over time came to be mods of specific forums, when there is so little activity here and so few members that just one global mod would easily be able to handle everything that goes on (which isn't much; as I said before, people on this forum seem to be well-behaved). This over-moderation is a trend of less active boards where the few active members express an urge to help even more with the community, and the admin (who doesn't want to lose the few active members he has) agrees. Not saying that is necessarily what happened on the GMG, but the over-moderation issue should be considered.
It's also worth noting that the old GMG also had the same moderation style, with several of the more active members having the "GMG" title affixed to the beginning of their names despite the fact that they didn't actually do much for the GMG.