Most Outlook users are used to the ability to send a message with importance (low, normal, high). In GMail, that feature was removed. Google's idea is that a new feature will help sort out the details of importance, called "Importance" in GMail. As you use GMail and Google Apps, it learns who you communicate with regularly, and assigns them higher priority than others. If you regularly send e-mail, hourly for example, to a colleague, GMail will learn this and help assign priority to their e-mail messages. When Linda in accounting contacts you on a daily basis with budgeting reports, GMail will know that it is important for you to receive Linda's e-mail. It then scrutinizes Linda's e-mail to you less than others. It learns that Linda's e-mails are important to you - because you receive them, never mark them as spam, and always open them up quickly.
This removes the need for someone to be able to flag, star, prioritize your e-mail FOR YOU. Let the system do the work, ON YOUR BEHALF. Google is all about helping YOU use the systems more efficiently - not someone always trying to game the system by marking their messages as "High Priority".