The problems you mention really do not relate to the threshold per se, but rather the way it is currently implemented. Let's go through your different points:
1) Button-pushing. You are correct, by weighing in topical comment scores, much-discussed troll stories have a certain likelihood to make it. Whether this is a good or a bad thing depends on what kind of site you want. I have always seen K5 as primarily a discussion site, where people go when they want to see all arguments about a given issue. By dumping stories that have lots of high-quality topical comments just because they are too controversial and don't make it within a short amount of time, we also stop the discussion (and make it mostly invisible). Note that the Adequacy story you mention would not have made it in that form, because of the large number of low-rated topical comments.
2) Time zones. Yes, the threshold could be higher and relative. A minimum time is not a good idea IMHO because it would hold back critical news, and K5 is slow enough as it is. By the way, given K5's nature, I would assume that many people's sleeping rhythms are far from regular, so people from all over the world are probably voting all the time.
3) Cliques and obsessive reloaders. Again, a higher threshold would mostly fix this problem, as far as it exists.
Your anecdotal evidence cited in 5) is a bit thin. I can see no such developments, and even if they exist, they do not necessarily relate to the threshold, for the exact reason you mention: The number of users is continually increasing and coming in from lots of different sources. How they behave is hard to predict. Your model can only be tested on empirical data, and has to prove its predictability on several sets of data. Otherwise it's just an opinion.
1) Dump everything. Not a good idea. The same thing you criticize -- button-pushing -- can backfire against you. If someone criticizes the "moral majority" on the site, his story may be quickly shot down by said majority. Much as I loathe deliberate trolling, the "dump everything" rule encourages groupthink.
2) Abolish threshold. You haven't been around so long. I've been here since early 2000 and I watched the inflation of stories in the queue. At some point, it got quite terrible. Stories were lingering for days or weeks and frequently you had a two-digit number of submissions pending. It was very unsatisfactory for authors and voters alike. People started to complain that they couldn't really be expected to vote on so many stories. ShouldExist is another Scoop site that worked without the threshold for a long time. There were about 30 stories in the queue last time I checked before they updated, many of them more than a year old. Yep, a year. On infoAnarchy I frequently push stories to the frontpage myself if they take too long.
3) Floor in the algorithm. This is really fine-tuning and the kind of stuff that's hard to explain a year after you've done it. I would not necessarily oppose this, but I would want to see actual evidence for harmfulness of the current solution first.
4) 24 hour story wait. No, for reasons explained above.
5) Change thresholds. Yes, this may well be worth doing. A relative threshold seems like a smart idea. This is what is generally used for real world direct democracy.
6) Rating stories. Won't really fix the problem on its own, you will still use some algorithm to decide whether a story has "stabilized" enough. But yes, this is a good idea that still needs to be tested in practice. It would be good if Rusty specified the whole system and submitted it to K5 before implementing it to get as much feedback as possible on potential problems.
7) Rating comments down. Rating should always be based on a comment's content, not on some desired story vote outcome. This is the kind of hacking that ultimately breaks moderation systems.
8) Leave system as it is. Well, not really -- we all want progress. But adjusting the threshold and then slowly moving to a new voting system seems like the smartest idea to me.
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