VIP — very important person — lets you create a separate group and notification priority for the contacts that matter the most to you. Sadly, it's currently limited to Mail.
*Update: Now that Apple Watch has shipped, having VIP as a system-level feature that not only Mail but Messages, Phone, FaceTime, and even App Store communications apps can filter through feels even more essential. *
Mail's VIP feature is great. I can "star" the people most important to me, thus enabling sound, vibration, and Lock screen notifications solely for them. It's a great way to make sure I'm alerted when I get email that matters, and avoid being interrupted when it doesn't. Sadly, VIP currently only exists as a feature in the Mail app. There's no way to do the same for Messages or any other communication app I have installed.
For iOS 9, I'd love to see VIP moved from Mail's settings to the system level for Contacts proper, so that I can make sure the people who are literally "very important" get through more easily, no matter how they choose to communicate with me.