Usually software would protect you with a confirmation pop-up: "Are you sure you want to __________ ?" Instead, GMail just let you do it. Then it added a yellow strip at the top, "Message deleted. [Undo]". So, instead of double-confirmation, GMail implemented Undo more thoroughly.
This was better in two ways:
(1) It wasn't a pop up! I hate pop-ups. I don't think anyone likes pop-ups.
(2) A computer asking you "Are you sure?" is as annoying as a human being questioning everything you say. It is nice every now in then, when it really was an accident. But most of the time, it isn't. Another metaphor is a mechanical device that is failing. You press the button, and it doesn't work. You have to press it again.
The whole approach breaks your flow, implies contempt, and by definition is frustrating.
The approach of "just let you do it and provide an undo" isn't recent anymore, and sadly, Google seems to be drifting away from it. I see more and more pop-ups and "Are you sure?" messages.