When Google introduced Gmail in 2004 with 1 GB of free storage, the GMail interface did not have a Delete button that could move email messages to trash with one click.
That was probably a wrong decision but it took almost two years for Google to add that "missing" Delete button to the GMail interface.
There's still no "official" GMail keyboard shortcut for deleting messages or moving them to trash (even Matt Cutts would like to see one.) You have to use the mouse to delete GMail messages, there's no way to delete GMail emails without taking the hand off your keyboard.
Or wait, there does exist a keyboard shortcut for deleting email messages in GMail though Google never told you about that "secret key" - it's called the "End" key that works perfect but only inside the Opera browser.
[Discovered this by accident when we hit the End key instead of the PageDown key and the email just disappeared with the message "The conversation has been moved to the Trash." Eureka!]
Unfortunately, this GMail Delete keyboard shortcut cannot be used in Firefox or IE but with Opera 9.1, it just feels more productive when you can delete a message with the End key.
Related GMail Reading:
How to Prevent Gmail Storage from filling up
How to Delete GMail Messages quickly