Obviously the minion icon is a April-1 joke, but the rest of the feature actually sounds like a pretty cool one. Imagine the last time you were on a long email chain. One where you gave your 2 cents, and there's no need for you to be in the email chain any longer. Someone else can remove you from the email chain by moving you to BCC, but there's no way for you to do it yourself. You basically have to beg for someone to remove you from the chain, which is too annoying so no one really does it.

Alternatively, you could create some sort of a filter, or just consciously delete all emails from the chain as they come, without reading them. This is manually tiresome, and it also introduces a new problem: If someone references you in the email chain, and you don't see it, they will get offended by you not replying.

This new gmail easter-egg addresses both of the problems above.

1. It tells your email client that you no longer wish to receive any emails in this chain.

2. It tells all participants in the email chain that you're leaving the conversation.

The 1st part above is purely technical, and can be easily done. But the 2nd part requires some UX development. Which might be why GMail launched this April-1 easter-egg. They are experimenting with the ideal UX for telling all participants in the email chain that someone has chosen to leave the conversation.

Obviously the presentation needs to be a lot more polished and professional. But easter egg might be a precursor for a real GMail feature that's planned to launch in the next year or so.

For anyone doubtful that Google would announce a permanent product/feature on April-1: Check out the very original gmail announcement: http://googlepress.blogspot.com/2004/04/google-gets-message-launches-gmail.html