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.