The system uses many criteria's to decide whether, which messages are most important. Some of the criteria's include, How frequently you open and/or respond to messages from a given sender, how often you read messages that contains keywords etc.
One of the most important feature is that if you come across a message that has been marked important that shouldn't have been, you can hit an arrow a message. On the other hand,if a message that shouldn't have been flagged gets sent to the "Everything Else" area, you can promote it. Gmail gets progressively smarter..LOL :-)
This feature would be available to users within a week's time and can be activated going to "Settings" tab of the Gmail options.