I've edited the wiki to read:

Main exists to encourage people to submit longer posts (on the order of a couple thousand words or more). Shorter posts may be appropriate for Main, if they are exceptionally high quality. Other factors that suggest you should post in Main include...

I do this without any pretense of representing some consensus from this thread; the consensus if anything seems to be that we should modify how the Main / Discussion distinction works at the level of site software. Unfortunately, that doesn't seem likely to happen any time soon.

So instead, the above is my attempt to explain the main purpose currently served by Main, influenced particularly by this comment and also Vaniver's comments. It's a rough patch, but I hope it's at least a slight improvement.