I'd be careful of comment thresholds at least to some extent. It's pretty well known across many of these services that if you reply quickly to a new story you're much more likely to get upvotes. Similarly with replying to a top post. If they need to be written quickly they often aren't as high quality or carefully considered, or may even fall back on some of the easy bad posting styles you're trying to eliminate. If you take the existing karma whoring incentives to do this and add to it that you may never even get read if you don't get in early that might cause a lot more people to play that game. And cause a drop off in participation from people who might only be able to respond to a thread when it's hours old.
I think in general you already have most of a filter in that bad comments get pushed down and the lower sections of comments seem to be read much less often.