Re: huh? - facepalm
>Aren't sending emails, posting to social networks and paying for something EXACTLY the sort of functions you need a cancel/undo for ..
You might well want that, but that would be a function of the service provider, not the OS. And the user would have to understand that their instant messaging system isn't instant, but has a lag of X seconds, suring which time they can 'Undo' their 'send'. Hmmm. Now you have the situation where 'send' now means 'send in a bit'... simple, heh?
My point was that if the main functions of a device are reading information or sending information, an 'Undo' button is of far less use than it is on a computer used for creating and editing content.
Now, the idea of conflating an 'Undo' button with a universal 'Back one step' button is not without its merits, but brings its own compromises. It is for reasons like this that UI/UX design is bloody complex, and being dogmatic can be counterproductive.