You can only show new comments, highlight them, even view them in flat mode. Threaded forum naturally separates subtopics while flat forum is just a big mess. Also old subthreads naturally die so the new stuff is usually in the new nodes. And you can also just view subtrees to follow certain discussion.

edit: In yesterday's discussion I posted some advantages of a discussion system based on tree structure https://news.ycombinator.com/item?id=5175660