I've never been a diehard follower of "SEO" - we just provide good content and the people come, that is all.
Those sites that replicate information from one source to another aren't really providing new or innovative content, will likely see some changes. Also, a lot of sites that have the same target audience where the forum population is over saturated, will probably take a hit as well.
On the other hand, I'm dealing with a site that has ripped our public content, word-for-word except they changed the user names and even have some of the posting dates set for dates that haven't even happened yet. (i.e. April 2012) - The problem here is that they are now ranking right below our main site when you search for a common phrase that was unique to our site only. What will Google do with this? Only time will tell. They have been ripping similar sites and even places like Yahoo! Answers, wtf?
Ok, my main point, I think having your website/forum/blog/whatever well organized is very important. Keeping out the fat is important, but it shouldn't overshadow the original content quality of what you provide to the public internet.