Medium-style side comments - http://aroc.github.io/side-comments-demo/#demo. More generally, I think that we could do some serious rethinking of how discussions are carried out over the internet. I wrote about it a little here, but those thoughts are very raw.
I think it'd be a good idea to keep a list of the ways we'd like to see LessWrong improve, sorted by popularity. Ie. email alerts for new responses.
So if you have an idea for how LessWrong could be better, post it in the comments. As people up/downvote, we'll get a sense for what the consensus opinions are.
I think there's a pretty good amount to be gained by improving LessWrong.
Note: I say "ways to improve" instead of "features" because "ways to improve" is more general.