The Gmail team has rolled out yet another feature that makes unsubscribing from mailing-lists easier. Now, if you mark a particular type of mails as spam from a mailing list, Gmail will prompt you an option to auto-unsubscribe from that mailing list.
To help solve this problem [the problem of unsubscribing], 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!
It should be noted that this feature works with "some messages" only.