I guess the thing may be that movies have the luxury of make-believe; that is, they have absolute freedom with these interfaces because as someone else pointed out, they don't actually have to function. Sure, their dialogs and warning messages are great to convey a single important message, but what about someone who needs to do something on the computer that reports the "lockdown" status of an area? And do you really want it displayed with a huge green bar every time you get into something that was previously protected? Aren't there some situations where discretion is preferable?

In the milder example of the email flying off the screen, who wants to sit around and wait for a few seconds watching these animations in everyday use? Sure, they're a neat one-off or demo, but when you have a stack of emails to pop off, nobody wants to sit around and wait for that cute little animation to finish. You can probably toggle it, but after the first few times, who would leave it on? It seems like something that would just make grandma's computer usage even slower.

I do think that we could probably look at movies more and try to cherry-pick some cool ideas for dialogs or animations, but in general movie widgets, like everything else in the movies, are larger-than-life. They wouldn't work if you tried to deploy them as a real usable UI.