I find the purely time-based ranking in forums helps there a lot. Reddit basically forces repeats over repeats of "old" topics, where a forum could handle it in in one large thread. Those can get unwieldly on large forums, but in many ways I still prefer a 50 page thread over 50 different posts somewhere in the subreddit history, with different titles. Especially if the forum community is good at maintaining roundup posts.

HN has that issue too in some ways, with threads about company X always repeating the same arguments, which in a forum at least somewhat could be channeled in extra discussions.