I have another gut feeling as the main target for panda has been an index of already flagged websites and not the entire web as general. Its just a theory but it might make a sense.
I think this theory has legs - it certainly fits in with my experience. Old site + forum. The content (non forum stuff) had a selection of fairly poorly written articles, but it all historically ranked really well. It was an old static HTML site that I never got round to upgrading so no new content was added in years (since maybe 2006). However, that was only 250-300 pages out of 40,000 (the rest were forum pages, which was pretty active).
Site got hit by the first international Panda roll out - organic referrals down 80% (traffic to the static content) but the forum referrals remained stable.
Not complaining about the traffic loss - the site was badly designed and poorly maintained so it's fair game. But I think it falls within the remit of you theory in that it was variably useful (some good content from expert authors, IBLs from the likes of Harvard Business School, BBC, and active forum) but also low quality signals (poor design, no new main content, some badly written articles, no contact info, some dodgy IBLs, etc). I fully accept that it probably raised a few flags due to high rankings for competitive terms but I would expect the realistically the design and selection of poor content caused the penalty.
I've just moved the content over to Wordpress, redesigned the entire site and ditched the forums (for the time being - they were too busy for me to maintain). I removed the crap articles (which were basically old school keyword rich SEO copy) and have been adding new (IMO reasonably well written) articles daily. 301'd all the redundant content to new versions or root.
I think the move to ditch the forum could have been a bad one, but from a business point of view it makes sense right now. I'm more interested to see how moving the content from the blatantly poorly designed HTML site to a professionally design WP theme site will make a change. Also pushing Twitter presence for the site and more effectively interlinked the content via various WP plugins.
New site only went live on 4 days ago - new URLs still haven't been indexed (other than new posts).
Will post back with results, but I'm reasonably confident that I addressed the majority of the issues that could have potentially caused the site to be hit by Panda. Whether or not a subsequent update will reflect this or not is another matter.
The site was relatively high traffic - 150k>200k visitors per month (now 30k>50k). Hoping to see at least some of a recovery.