I worked at Google 2006–2010 and I remember it as being unique in many ways. While I was there, though, one particular thing which distinguished the core product—Google Search—from other similarly popular web properties was that it had a stated aim of reducing "time on site per visit".
For Google a top-level measure of its success as a search engine is how quickly it can provide you with great results for your query and send you on your merry way. A measure of how well Google's doing its job is how quickly it can get you the result you're looking for. The better it does, the happier you are as a user, the more likely you are to use the product, and the more opportunity to show you relevant ads.
In order to make sure that you're not just being sent away for the sake of it, though, Google decided to measure both "short clicks" and "long clicks". Let's say I click on a Google Search result, find it useless, and immediately click the Back button. Google measures that as a "short click", ie. I wasn't away for very long. The implicit signal is that the search result didn't answer my query well. Google takes note of this.
In contrast, a "long click" is one where I navigate to a search result and don't come back straightaway. The implication is that I found what I was looking for.
Knowing this context it was interesting for me to see for the first time this feature on google.com. Search for [hunter walk twitter] to find Hunter's Twitter account. Click on the #1 result, then click your back button. You'll be offered this screen:
The feature's at least six months old but is the first time that I've seen such a direct manifestation of short click feedback in the user interface.
I'm impressed at the option to customize my search results this way. Never forget, in fact, just how darned impressive Google Search is. It's surprising, though, that this is such a coarse setting. Offering to remove all twitter.com results for all queries henceforth seems a little severe as a proposed reaction to a single short click for a single query.