story
As the coiner of the term, you may want to update the wikipedia article which, after listing the basic requirements says: "As these services have matured and grown more popular, they have added extra features such as ratings and comments on bookmarks".
So there seems to be a convergence of terms and services here.
I think the major difference between social bookmarking sites (of which I see HN as one, and you obviously don't) versus a forum is that on a forum the vast majority of the topics is not started with a link.
On HN the vast majority of the topics is started with a link.