I would love a way to collapse the parent of a comment (or all it's children) without having to scroll up and find it by carefully tracking indentation. Some sites do this by drawing the tree of comments directly with vertical lines going down beside siblings, which can be clicked to collapse. That's a much bigger target.
I agree that navigation is an issue, especially with large threads. From my perspective it's strictly a UI question. I don't want to spoil HN's minimalism.
I've been thinking of experimenting with links in the comment line (the same place that the timestamp and "[-]" appear) which say things like "up" (to go to parent) and "next" to go to next sibling.
Hmm - but what if you want to retrace your steps? Say you're reading child #5 of a very large subthread, you click 'up' to go to the parent, which scrolls to the top of that subthread—and now you change your mind and want to go back to where you were. This seems like what the back button is for.
I'm all for not breaking the back button, believe me, but I fear that people may have very different notions of what that means.
