suggy - 7:31 am on Jul 2, 2011 (gmt 0)
Tedster -- it also means that Google doesn't need to collect lots of user feedback data about individual pages (where sample size would be a problem outside big searches). It doesn't need to know that YOUR page has a high bounce rate or low CTR, only that it looks like one that would.
After reading a paper about how CTR is a pretty good predictor of bounce, I think Google feels it can afford to be fairly bullish/ aggresive about this classification and filtering.
As far as diversity in search phrases goes, it's pretty clear from the suggest tool and their rewriting of search terms, that Google's getting pretty good at 'understanding' what we want (or at least, thinks so!).