Currently when a SO post has more than some 5 or 6 comments the rest of them are hidden and you have to click a link to show them all. It would be nice if there was an option that all the comments are shown all the time. (Or even better - that you could specify the hiding threshold).
This would have no effect on the vast majority of posts (those that have five or fewer comments, and those that you just don't care about reading comments of). It would be actively bad for posts that have many comments you don't care about, because it would increase scrolling.
For those few posts where it would help you, that's adding a lot of complexity and spending a lot of dev time to save a few "show all comments" clicks. Comments are designed to be second-class content, after all.