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Since BigDaddy, many have been screaming about the decreasing quality of Google's index.  Now let's get a few things straight.  Some SEOs (ahem, spammers) may be the last demographic that you want to ask about index quality; but even the whitest of white hats are noticing.  One of earliest cited problems is that Google is kicking many pages out of the main index and into the supplemental index.  Many web masters complained, and Google listened — at least temporarily.  Now Matt Cutts says something ambiguous like "I believe any changes on the [June] 27th were refreshing data used by an existing algorithm," and at least that particular problem seems to be recurring.  Lots of valuable individual pages as well as entire sites are supplemental again.

From Google's Help Center:
A supplemental result is just like a regular web result, except that it's pulled from our supplemental index. We're able to place fewer restraints on sites that we crawl for this supplemental index than we do on sites that are crawled for our main index.

It appears to me that Google, again, has gone a bit trigger-happy on asserting that pages are spam.  For example, there are many pages that, in all honesty, don't need much content.  A simple example of this would be a repair shop that repairs a few hundred items, and has a page with a brief description of the item and repairs, as well as a price.  The items are typically non-competitive keywords.  Unfortunately, this type of page can look a lot like a doorway page.  Unfortunately, I believe that's the trap Google is falling into.  Even white hats are spammers now, apparently.
 
See this thread on DigitalPoint Forums.