HOW TO: Move Message to Trash: No Delete Shortcut key in GMail
We have a GMail productivity tip here for users who get too many email messages per day. Read 
and delete messages by making minimum use of the mouse.
When Google launched GMail with 1GB of free storage, the GMail interface had neither a Delete Button nor any sort of Delete keyboard shortcut to move email messages to the trash.
Maybe, Google expected that with that kind of "infinite" storage limit, people would never want to delete email messages and instead put less-important messages in the Archives. Unfortunately, that wasn’t the case, people started reaching the maximum storage limit and forced by public outcry [read, GreaseMonkey], Google did introduce a GMail Delete Button. How to Move Message to Trash: No Delete Shortcut key in GMail ยป Digital Inspiration