A good color man should explain the action, especially in football. I want to hear X's and O's, the concepts behind each plays, things I might not know as a layman. Gruden is very good at explaining what is happening on the field, so was Janikowski, and Mike Maycock too. Trent Dilfer is another favorite of mine, whenever he gets to do games.
I don't tend to listen for people to teach me anything, I know the rules and players of the game well enough. I just don't want to find myself disagreeing or being annoyed with what they're saying more than anything.