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.
Recent Discussion
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)
I have now tried this twice, and in neither case did I leave it on for more than about 10 minutes. Votes and poster name are not sufficient for me to know if I want to read a comment, so I have to pretty much roll-over or expand everything.
Skimming and scrolling is _SO MUCH_ easier and faster than that, on both mobile (note: I tend to use 8" tablet more than actual tiny phone) and desktop. Either hide the not-good-enough comment entirely so I don't have to think about it, or show enough (maybe first 5 lines) for me to decide if I want to read more.
Or leave it optional - this may simply be a strong preference of mine that others don't share. Note: calling it "experimental features" is a mistake. Call it "(experimental) collapse most comments by default". This makes it possible to have multiple experiments running, and allows those of us who hate this one to still participate in other experimental features. The even better middle ground is to have a checkbox for this feature, AND a checkbox for "opt in to new experimental features", which will cause any future experiments to be turned on until the user turns that experiment off.
It's good in theory but probably not practical for us to make separate experimental feature checkboxes for every feature set. Experimental features change pretty rapidly, might change names or otherwise get reworked in a way that makes individual checkboxes hard to maintain. (Note that right now the experimental features also do things like show you a new link to the /shortform page on the sidebar.)
But you can probably get what you want by toggling the nearby checkbox "Do not collapse comments on Posts" and/or "Do not collapse comments (on home page)"
(A week ago we also broke the singleLineComments in a way that made them much more annoying, which will hopefully fixed within a day)