First post, long time reader.
For me thin content would be system driven or CMS driven such as pagination pages and tag pages. Both seem to be an areas where a lot of pages were globed together. Liberal usage of tags formed (UGLY) lists where the KW is the header and not much else is there except page titles and links.
As I looked around at other large sites that had lost traffic this was somewhat common.
I removed several thousand tag pages and have been waiting on G to remove them from the index. So far only down about 200 since the 27th.
I have also noticed my index rate is slower than before. Perhaps because the tag and thin pages reduced my sites authority?!?
Lastly, in SERPs where I did rank first page or high second page. Pages or sites with very high Alexa, yet old and pages attached to giant what I call tier one sites now rank with pages that are often times not much more than signatures in older forums that have my KW as the title. The oldest I have seen so far is 2006 (stale!)
I have also seen door way pages and domain kw URLs move ahead as well as aged old sites that have not ranked for years but now are tops.
I can agree with the thin content being a factor to potentially why I might drop (15-20 percent traffic) but to have junk pop up high is very odd at the same time.
Lastly, my feel is the more competitive terms tanked the most if there is a site-wide penalty or tweaked algo where my thin content caused me to drop. More competitive, more dropping. Less competitive less drop. Long tails are OK.
Will comment when all removed pages are no longer in the index or if I have something of value to add.
All the best