Favorite Tags, auto-learn
It would be cool feature if Favorite Tags could be pre-populated based upon the a users commonly used tags. I know veteran SO users that don't even know what Favorite Tags are used for and are surprised when they see I have highlighted questions which reflect my favorites list. There could be a feature that allows users to select which of their common tags they'd like to use as favorites based of a given threshold (used xx times) or selected via picklist.
It would be great to activate this feature when a user reaches a certain milestone - earned a badge for a given tag, hits xxx reputation level, etc. This would be a great way of introducing users to the favorites capability.