dickbaker - 9:53 pm on May 15, 2011 (gmt 0)
These suggest user engagement with the site, which is something that Google likes. Again, no single factor wins, but some combinations may outweigh other combinations. Here, user-engagement (to the degree it's happening), may be more important to Google than organization.
It would be interesting to see how many sites with multiple purposes have been hit versus not hit.
In the case of this particular site, there's literally just a handful of posts in the forum (meaning almost no user engagement), almost nothing in the classifieds, and little in other areas where the user would be engaged.
I see successful forums that rank well with online stores, advertising, articles and other content that make them more than a single-focus site. That's just one area where I think your user engagement theory rings true.