One thing I always liked was the old discussion forums which had subforums for different subjects and then the posts in each subforum were ordered by most recent comment.
The result is similar to voting. Things people are interested in discussing on stay on the front page, things they're not fall off. But then you don't need "gravity" to downrank old posts with many votes, because when people lose interest they stop commenting. Which also means something stays visible as long as people stay interested in it. Whereas "gravity" encourages discussions to be cut short because it's almost impossible for any post to remain visible for more than a day or two, even it was still actively being discussed.
> Things people are interested in discussing on stay on the front page, things they're not fall off.
I like this. So currently on the homepage, and individual user (nest) pages the list of content has two sort options, "hot", and "new". The "hot" sort takes into account the message activity of the post similar to what you described. The effect is that active posts bubble to the top, and even old posts can become current again (anti-gravity) if the activity picks up once more. I think about how humans interact in real-life group discussions, where there is no "voting" on a discussion happening. The "voting" in real-life is in the form of the size of the discussion and how many people it attracts. If you put 100 ppl in a room and gave out a topic, the most interesting discussions would naturally form and people would be attracted to those.
The result is similar to voting. Things people are interested in discussing on stay on the front page, things they're not fall off. But then you don't need "gravity" to downrank old posts with many votes, because when people lose interest they stop commenting. Which also means something stays visible as long as people stay interested in it. Whereas "gravity" encourages discussions to be cut short because it's almost impossible for any post to remain visible for more than a day or two, even it was still actively being discussed.