Gmail helps you unsubscribe from mailing lists you no longer want to be on
I've been guilty of it, and if you subscribe to a lot of mailing lists, I'd be willing to be you have been too. Guilty of what? Of marking legitimate mailing list email as spam as a shortcut way to unsubscribe. To be fair, most of the time when I've done this, it's because the mailing list in question makes unsubscribing difficult. But sometimes it's just sheer laziness.
The problem with marking legitimate messages as spam is that it messes with Google's spam detection algorithms. So Google has gotten smart, and when it detects that you've marked a legitimate mailing list as spam, it will now offer to properly unsubscribe you from the mailing list automatically.
Google reasonably says that they can't provide the unsubscribe option for all mailing lists, and that it will be careful not to offer the unsubscribe option for lists that are known to be owned by spammers.
This is a great example of a web service going the extra mile to use the information it has to make life easier on its users.