Even though HN recently added the
prev/
next buttons, I find it much more convenient being able to click on the indentation bar in Reddit to collapse the comment that the bar drops from.
Specifically, I often decide partway down a thread that I've read enough of it and want to move to the next one—on HN, this entails scrolling up to the parent comment of that thread to click its [-] or next button, disrupting the flow of reading. The root/parent buttons are supposed to do this for you, but they're only useful if the thread in question is top-level or only one comment deep, which is decreasingly the case with increasingly popular posts.
The click-to-collapse in Reddit is sadly only implemented by some subreddit themes (and in the redesign), but in the default theme the visible bar is still useful for clarifying the level of indentation of a comment following a deeper thread that puts its parent out of view.
Also, is there an accepted name for this sort of 'indentation line'? New Reddit gives them the 'threadline' class, a name I don't see anywhere else. There's something similar in IDEs for collapsing indented code blocks, and the vertical lines in Win32-like tree views seem related, but I can't find any names for those, either. Searching only reveals a few post-redesign Reddit threads complaining about the click-to-collapse being unintuitive. But considering all the 'HN secrets' threads we have here, that seems to make it a perfect fit.