The old boards used to “bump” a thread to the top when a comment was added. This was nice for keeping track of new comments, but sometimes got in the way as really, really old threads would be hauled up from the bottom this way.
I’m not missing that feature much since I use the “comments” button to keep track of new comments, and it works really well. There’s also a “posts” button that tells me about new posts. I don’t know how much of that is available to non-moderating users.