To me, a forum is about topics and people can discuss them at their own leisure, bumping them in the process. In contrast slashdot, reddit, HN are more like "what topics are currently on", and if only few people are interested in a topic and visit the site apart from each other, no story they are interested in will make it to the front page, and they can't even discuss it "slowly, sometimes", like they could on a forum with a search function.

On reddit you at least get informed of replies to your comments, but discussions older than X are automatically closed, so that's not enough use to shift the balance for me.. I remember when a topic being locked felt like something rather grave, where either mods or users fucked up - now it's built-in. And sometimes discussion even gets substituted with clicking buttons.. which is also not about the topics, but another form of using outside force (something other than words) on words.