We must have used keyboard shortcut for browsing through gmail.
( You have to enable it in settings to use it actually !! )

There are some really useful shortcuts (which I have given below for your reference)

c - compose a new message
n - goes to next message
p - goes to previous message
! - reports as a spam message
r - reply to the message
a - 'reply to all'
f - forward that message
But what is the key for the Delete in the Gmail ??

It is not specified anywhere in the google help pages...

But someone in blogging community has found it to be # ( that is Shift+3 )

This moves the message to trash immediately!!
So if you are using the Gmail mail keyboard shortcuts, they you can use this also and increase your productivity

There are many others keyboard shortcuts also. You can read it from the Google website.
But there are really too much to remember and to use....