One thing we do to try to give more visibility to really useful threads is to make them "sticky." A "sticky" thread appears at the top of the subforum index, so it's easier to find. We don't want any one subforum to have too many sticky threads, though, or the distinction won't have any value.
The way it works is usually that someone recommends some really productive thread to the moderators, or maybe one of the moderators notices such a thread, and the moderators look it over and make a subjective judgment based on experience of the forum about how interesting it is likely to be to a wide audience and what the signal to noise ratio is. It's often a decision made by just one or two moderators. We don't vote on it and don't try to apply any quantitative standards, and it usually works out well.
.::Gary Rowan Higgins
A comfort zone is a wonderful place. But nothing ever grows there.