technologically, the high importance flag is a header set in the email. you will not normally see the header info (it requires you to change settings normally, and would look something like x-importance.) email programs will respond to this header.
Im sure other email programs this is true, at least for apple mail, you can have the program watch for keywords, and using rules can have that do other things. for example, on my mac, any robotics related items are colored red.