If you have notifications set to everything you will have a lot of messages marked as important. Try altering your notification settings.
Leading indicators used to determine importance are:
- You have been mentioned
- The chat is between you and just one other person
- The chat is a part of your favourite list
- You have notifications for that chat set to 'all messages'
- How often you participate in that chat
- How close you are to the person talking in that conversation. This is based on a calculated relationship strength with each of your contacts.