A few quick observations from one affected site:
1. Rankings lost across most but not all keywords, some falling about 30 places, some more than 100.
2. Still ranks #1 for its site name, with all its 8 sitelinks intact.
3. Most of the drops centered around pages that naturally don't lend themselves to lots of content. In other words, the pages may look "thin" but only because there's not much to say via text about the actual content. This could be applied to various types of content including images, online games, downloadable or printable items - anything that might only contain a few lines of how-to-download or how-to-play type of content, near the main non-text content. Starting to think I'll need to create ehow type content that is useless but must-have filler to avoid being filtered.
4. Some of these might have some duplicate issues as some of it has been scraped by others (affected site's content is original), but I don't think there's enough of that going on to be the main culprit - still, it could be a factor.
5. Several other sites (5 total) that are essentially built in the same way, with the same types of content, but for a different niche, were not affected at all. The affected one is the largest and most high-quality of them all. It's the one that garners the most attention and traffic. It does have a lot of funky backlinks but they are all natural. Funky sites link to it all on their own, with no requests being made and no link building of any kind being done. No paid links (to or from) at all.
6. Oh and there are some pages that are "natural dups", meaning everyone who has these types of pages would have no choice but to be duplicating everyone else - a good example might be a song's lyrics. I suppose UGC or more bogus content could be added but that just dilutes the user experience, I think, still, it may need to be done anyway.
7. Even if all the potential dup issues were removed from the equation that wouldn't explain all the drops. So, while that might be a factor, it's not the only one.
8. And I still don't understand why one niche of its type got hit but not the others. This might be my best hope of analyzing though, since I'll be able to compare link profiles, onpage content, dups, etc. on very similar sites. Will report on anything else I find.