Ever get an email from a legit mailing list but went ahead and marked it as spam anyway because you just don't want it? Gmail has smartened up to this common behaviour, offering to automatically unsubscribe you from certain messages.
From Gmail's help pages:
We don't think you should be burdened with managing messages you don't want to receive. We do our best to put messages in Spam when we're pretty sure you won't want or need them. But everyone has different preferences about the mail they want to see. You may not want to read any messages sent by a certain company or mailing list, while another Gmail user finds these same messages to be valuable.
To help solve this problem, we're providing you with an unsubscribe tool for some messages. You'll see the unsubscribe tool when you mark a message from particular types of mailing lists as spam. If the particular message is a misuse of a mailing list you like to receive, you can Report spam as usual. But if you never want to receive another message or newsletter from that list again, click Unsubscribe instead. We'll send a request to the sender that your email address be removed from the list. It's that simple!
Basically, when you're annoyed enough to mark a message as spam but not enough to go and unsubscribe from the mailing list, Gmail now offers to do the more time-consuming part for you. They won't offer to unsubscribe you from just any list, though, and keep in mind that it can take a few days to process your unsubscribe request. Still, it's a really nice feature to provide. Thanks Gregory!