Why don't you consider a sort of a point sytsem, like Amazon or has for instance where ranchers can vote up or down a particular post and the post can get 'hidden' depending on the number of thumbs down or up for this particular forum? I think if you let a particular topic reach a certain number of posts, people will feel some unfairness when you suddenly lock it. Either close it from the get go(it was quite easy to predict from the very first post where the windows vs. other OS was going to set into motion) or then acknowledge that it is just MD and allow it to grow no matter what kind of crap people are saying. Reading your reason for closing the windows thread even I was a bit confused. Was is closed because it had meaning? or was it closed because it was being injected with meaningless crapp where people were trying to have a meaningful discussion?
as far as "closing it from the get go or leaving it forever" - that's just not possible.
a lot of threads start out just fine, and then deteriorate. Some threads look like they're not gonna last through a second post, and turn out to be very interesting, fun and worthwhile.
there is no way to tell every time from post #1 which way any thread will go. The moderators are all volunteers who do their best. Sometimes we make mistakes. Sometimes we do things that another mod would not have done. sometimes, we do the unpopular thing, but the probably CORRECT thing.
IF a moderator has the time and inclination, he or she can go back and prune out the objectionable parts of a thread, but often that is more of a hassle than it's worth.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors