I have a very large forum (just hit 10 million posts), and have noticed some SEO issues recently. While reviewing my site in Google Webmaster Tools, I found it was complaining about 5,600 pages with "duplicate meta descriptions". Here's a sample of the full report:
I realized there are two reasons a page would be flagged here: either the same thread has been renamed, which changes the URL (and for whatever reason, Google is ignoring the redirect), OR multiple threads have the first post which contain the exact same content.
Of course, I'm not quite sure what an alternative method would look like, how exactly this could be "fixed", or just how much of a concern "duplicate meta descriptions" actually are (there appears to be some healthy debate about this among SEO experts). Regardless, I wanted to bring it up.