We recently added an "beta testing" option for LessWrong. If you go to your account page, you'll see a checkbox for "Opt into experimental features".
Right now, the primary feature under development that isn't yet released to the public is Single Line Comments. This is an attempt to fit more overall comments in some areas.
For example, recent discussion now looks like this, where each post loads 4 comments (and highlights them if you haven't read them), but only shows significant amounts of text from the most recent comment:
You can click on a recent discussion item to mark that item as 'read', and make the green line go away.
Posts with 50+ Comments
On posts with 50 or more comments, comments below 10 karma will appear as a single-line comment:
Mousing over a SingleLineComment will show a hovercard with a preview of the comment.
Clicking on a thread will fully expand all of the children of the comment that you clicked on. Doing a search (Control+F, or Command+F) will also expand all comments (so that you won't run into annoying things where you try to search but the comment isn't displaying all the text so you can't find the quote you're looking for.
This is all still under developed. Admins of the site have been using it for the past month or two to see how it works, and tweaking it until it felt usable. It's now at a point where it seemed good to let other LW users try it out and see if it seems like an improvement.
(Before releasing it publicly, we plan to build some sort of safety-valve-checkbox where you can turn it off easily)