Discussion forum: number of posts - number of authors - date of the first post
The extended snippet categorizes the search result and indicates its potential usefulness: for example, a thread that has a single post or a single author is not very useful.
Other search results that include special metadata:
* videos show a thumbnail, the duration and sometimes the average rating (on a related note, Google no longer lets you play videos inline)
* Google Books results show the author, the published date, an important category and the number of pages
Google slowly categorizes the documents from its index and starts to show additional information relevant for each category. We can expect to see extended snippets for blogs, news sites, shopping sites, scholar papers and even a way to restrict the results to a certain category. An experiment from last year grouped the search results in different categories: references, reviews, stores etc.
For now, the extended snippet for discussion boards is not live for everyone, but you can change your Google cookie to be a part of the experiment.