Here's an example which I hope will prove helpful to the Google Engineers in theory reading and debugging these issues raised in this thread.

Right now my site has one of only a handful of reviews on the internet of the upcoming movie called "Source Code".  Here's that review:
http://bit.ly/etXXF3

2 week ago searching for "Source Code Movie Review" would have included our review somewhere on the first page of results for that term.  Now instead what you get is this:
http://www.google.com/#sclient=psy&hl=en&safe=active&q=source+code+movie+review&aq=f&aqi=&aql=&oq=&pbx=1&bav=on.2,or.r_gc.r_pw.&fp=eb8390001e6cd53f

Of the ten websites listed on the first page of results, only 2 of them are actual reviews of the movie "Source Code".  One is a RottenTomatoes listing of Source Code reviews.

The other 7 are basically garbage, keyword stuffing, and spam meant to work their way into the "Source Code Movie Review" keyword without actually providing any kind of review of the film. The first two listings don't even contain any content and are the very definition of "thin content".  They not only don't have a review of the film, they don't really have anything on their pages at all.

So only 3/10 results there are useful at all and most of the first results are completely useless thin content.  Two weeks ago, at least 4/10 (if not more) would have been good content which actually related to the keyword, since our review would have been on that page.

Our review actually is on the first page when you search for the term with Bing:
http://www.bing.com/search?q=source+code+movie+review&go=&form=QBLH&qs=n&sk=&sc=0-9
And overall there is a much higher percentage of actual "Source Code" reviews than there is in the Google results for the same term.

Hopefully that provides a good example for the Google team to analyze what's gone wrong.  Waiting patiently for the fix, holding on as long as we can in the meantime.