This is an unfair comment. Think about all the features gmail has that aren't buttons in the UI. Would you really like 1) Buttons for all of these 2) some pop-up or otherwise annoying message coming up for each of these features when your trying to write/read an email?

I think that's the challenge of good UI - being visually informative without overloading the user.

Subtlety is a difficult thing to achieve in any field. Silence, however, is anything but subtle.