If you use Google's E-mail service (directly or through Google Apps), you no longer own your E-mail. It is the property of an advertising company that will do with it whatever it sees fit, including reading it, learning from it, modifying it and hiding some of it from you.

This needs to be explained to people, as most still think that Gmail just displays all of your private E-mail with a nice interface, FOR FREE!

The "FOR FREE" part is really the killer, I found that people are amazingly resistant to the idea of paying anything for E-mail, even if compared to their other utilities (power, water, internet, TV?) it is extremely cheap.