I would like to suggest modifying the collapse button to feature a right arrow symbol (π) on the left side of the name to indicate a collapsed thread, and a longer down arrow symbol (π) for an uncollapsed thread. The thread delimiter line would go up just to the symbol. This change would replace the current two diagonal arrows that are located to the right of the votes, which I hadn't even noticed before today, and I thought that functionality was missing.
I'd also suggest enhancing the visibility of the thread delimiter line. I suggest making it grey and of the same thickness for most of its length, and then thickening it, and increasing its opacity, and adding color only at the level of the comment.