Doesn't matter much to me since I rarely participate here, but I'm getting spammed (well, I was until I just shut off PMs ) via PM by brand new members with no posts to their name. You might consider disallowing members with no posts from using the PM system. My 2 cents.
I wish we could do that also. But I would go with something more like 1 week timeframe, 25 posts, and a certain reputation level. Only shawn can change that so it might be awhile.
It's been suggested a few times already - not likely to happen. They'd just spam discussion threads more, mods wouldn't be able to reach them if we needed to, they couldn't contact us with questions, a lot of posts now require that you be able to PM b/c of the new rules to keep BST clean anyway, etc.