I've just added some potentially cool functionality to CAREO. Every object in the repository now has a link to its own Wiki page, where a freeform discussion or whatever can take place.
I'm not sure if people would actually use it. I think it's different enough from a traditional threaded discussion to warrant inclusion of both solutions.
In my head, I'm picturing the Wiki pages becoming more teaching-oriented than a threaded discussion could. Something more like the Wikipedia.
The cool thing about the wiki pages (apart from being more freeform) is the automatic cross-linking of pages. This could have the potential to make a richly networked set of resources wrapped around learning resources.
As an aside, I'm using phpWiki to provide the wiki functionality. I find it more full-featured than UseMod, and it supports creating wiki pages with full URLs as page names (that's all this feature is based on).