Here's a simple one. Do you ever find it annoying that in order to Reply to a message, you have to scroll all the way to the bottom of a message to click the "Reply" link? While reading a message, simply press the "r" key, and the Reply editing box will open ready for you to compose your reply! Oh, and pressing the "f" key will also open the "Forward" editing box!
To enable keyboard shortcuts, click on the Settings link at the top right of the screen, and click on the "General" tab. Look for the "Keyboard Shortcuts" entry and click on the "Keyboard shortcuts on" radio button.