http://www.google.com/quality_form?q=foo
The problem is that it gets polluted by lots of people who have no clue that Google is not the Internet, or (for that matter) their neighborhood handyman that they found through Google and who did an awesome job repairing their windows. A lot of people don't make a distinction between "I found what I was looking for" and "What I was looking for worked out for me", which makes a lot of the feedback a little less than useful.
This thread is as good a place as any - I'm guessing that the URL will get passed around, the appropriate teams will read it and adjust their algorithms, and if DuckDuckGo gets to improve their algorithms too, great - it's one more good search engine that people can use.
Also, google is one of the worst companies, if not the worst company, at dealing with user feedback. You can't just expect people to give you feedback but never return the favor in any way. People have the feeling that giving feedback to google is like throwing things into a black hole - like talking to a machine - if you know no one will answer you and you will never know if anyone even read your feedback, not even a little thank you note or a clue that the feedback was useful, there isn't much incentive to give feedback, now is there?
As a result, I enjoy reporting issues to Google Maps, because I know they will be addressed. Maps are dealing with a lot more tangible and unchanging dataset than Search, though.