When I just started, I quickly setup some filters to categorize all mail into three categories: those where I would be one of the few people in To, or those from my boss, or those that are High Priority would go into first category, most others will be in second and mail group etc. will go into third.

The problem with the system is that people who send you email are not aware of it :), and very often violate those rules. Apart from some form of AI gradually adjusting rules over the time, I'm not sure what would be a good solution.

Or just turn on Priority Inbox in Gmail.