I've been using Gmail for 185 days now. And I stumbled into what I think is the biggest Gmail time saver yesterday—keyboard shortcuts.
It was disabled by default and you have to go to your settings page to enable it. But once enabled, you can navigate your mails, in two different views, with the keyboard.
The j key takes you to the next thread. The k key takes you to the previous thread. The Enter key takes you from the summary view to the detail view. The u key takes you back.
(Just to show you how "web applications" takes the state of the art GUI development back for twenty years. (Sorry I couldn't resist the urge to take a cheap shot at "web applications." (And I think I understand what Bill Gates meant when he said "Have you ever heard of Windows?")))