Actually I think Windows has always had a little popup in the bottom right which lets you cancel within about five minutes. But sure, you might happen to step away momentarily during that time.

Thankfully browsers and editors now save and restore their last state, so an automatic reboot doesn't enrage me like it did in the WinXP days.

While that modal does exist, Windows for some reason does not give it always-on-top status, so if you have anything fullscreen running you'll never see it. There also seems to be a limit to the number of times you're allowed to delay a reboot, though I'm not entirely sure on that.

On Windows Server 2012, if there are pending updates as soon as you RDP in it will start a 15 minute countdown with no option to cancel (at least no obvious one I could find). That's ridiculous.

Agreed. I've lost work before due to this behavior. Granted, I should be more diligent with saving my work.