I think the forums do a pretty good job of accomplishing most of those tasks as it is. The "Ask a Question" forum has a wide variety of solutions to common development problems. The forums house many extensions and behaviors, and several artists share graphics, music, etc. either in the art section or on their own site linked to in the art section.
The main advantage to StencylForge that I saw was that it streamlined the process of downloading and installing new assets. It's not hard to import resources, but for a total beginner it made building a library of behaviors just a little bit easier.
I think just a little reorganization of the forums could serve just as well as StencylForge. For example, "Game Art" could be split into requests and "shared assets", so that assets could be more easily found. Or, maybe "Shared Resources" could be split into the categories StencylForge had, with an additional "requests" sub-forum so that the resources areas could be easily browsed without requests "getting in the way".