A discussion forum administrator posted questions about how to improve his forum in the eyes of Google, the question was posted in a Google Webmaster Help forum and Google's John Mueller responded to it in detail.
Reading the advice, at least to me, shows how Google has grave concerns with content within forums. All the advice is around guarding the forum from low quality content as much as you can. John Mueller wrote:
Another thing you didn't mention, but which is common with forums is low-quality user-generated content. If you have ways of recognizing this kind of content, and blocking it from indexing, it can make it much easier for algorithms to review the overall quality of your website. The same methods can be used to block forum spam from being indexed for your forum. Depending on the forum, there might be different ways of recognizing that automatically, but it's generally worth finding automated ways to help you keep things clean & high-quality, especially when a site consists of mostly user-generated content.
Google has had a sensitive view of user generated content for years and years now. With Panda and all the other algorithms and manual actions, this type of content can be a scary thing.
We actually killed our online discussion forum here a year or so ago. Plus, I am strongly considering making major changes to our comments here. I delete all the pure spam stuff but some of the comments go way off topic.
Anyway, John offers more advice on forums and Google in the thread.
Forum discussion at Google Webmaster Help.