FYI: Bing is finding old threads that Google is not. [more inside]
For the big pile of stuff: How about a wikipedia-style "Mefi posts that link to this post" box on the side of each thread (that has been linked back to)? So when someone links back to a post, that old post now links forward to the new thread. Especially useful if you search and find an older thread, and would like also to read the subsequent threads which say "previously discussed on MetaFilter..." Ideally, it would pick up on comments as well as posts that link to a specific thread (not to a specific comment in the thread, though).
Maybe it's just that the server didn't refresh or something else, but I just posted a message to an old thread (February 22, 2002), with some updated information, and it doesn't seem to have made it to the front page. Is there a limit to how far back the "Recent comments (on 20 threads)" looks?