This is exactly why HN/Reddit format of tree-structured discussions is superior to linear phpBB forums. They allow for segregating subtopics (on a long forum thread, there are always subtopics and offtopics), letting you ignore the things that are not relevant to you.

Were I setting up a discussion forum today, I'd immediately look for something halfway between HN and Reddit. Reddit itself is too much of a PITA to host (we tried in our Hackerspace, but didn't go too far); HN has too little markup and no built-in functionality for storing notifications about replies.