(Turning this answer into a feature request.)
Stack Overflow is a repository of great and not-so-great answers. Three major causes for the not-so-great is misleading question titles, outdated answers, and poorly-answered duplicates with no connection to a better original question.
Anecdotally, today, a new user who had arrived from Google posted this "not an answer" underneath a question of mine:
@Mewp - 2 years after you answered the question, I ended up here (via goggle) looking for a solution on mod rewrite and the tutorial site you linked to is GONE!! Now what?? [...] So annoying that this ranks so highly in google for a lot of technical searches but unfortunately the site is full of unhelpful answers!
While it can be debated whether that question should exist in the first place as it is a resource request, the issue is widespread and goes far beyond resource requests. As a solution, Stack Overflow should consider voluntarily withdrawing questions from the Google index that gather some form of negative feedback.
Ways to collect feedback could be:
Ask the googlers. When a user comes in from Google, ask them something like
We see you found this question while searching for
how to flobber the gargle. Was it helpful and relevant to your query? Yes / No
If a question receives too many "unhelpful" votes, either remove it from the Google index through robots.txt, or add it to an "unhelpful" review queue specialized on editing titles and such, or both.
Use the existing anonymous feedback data (Jeff Atwood himself has indicated that anonymous feedback could be useful for the cause) and exclude questions from the index that get overwhelmingly bad feedback.
When a user comes from Google, measure the time they spend on the page. If they leave it immediately with no action at all, interpret that as an "unhelpful" vote.
Also, perhaps questions with two or more close votes (except of course duplicate votes) should be withdrawn from the Google index. That would go a long way towards cleaning the search results from trivial garbage, too.