1. Admins could create links. This would really help the situation where an admin closes a thread because of a duplicate topic. If the "similar thread" mentioned the other topic, they would probably be easier to find.
2. You could allow users to create links to help out the website, perhaps for free schwag. I'm thinking of something like Amazon's mechanical turk, where you just present two threads to a user and ask if how much they're related. And if the user answers enough questions, they can earn credits for a print or something.
Since there are threads that have lots of images, text-based stuff probably will never be the best way.