I've always felt that they are movies that are pretty much purely entertainment, with no real message or need to think about anything. Movies during which you can shut off your brain and have a good time. Men in Black, The Rock, Blade, and American Pie would be movies I'd consider good "popcorn flicks". Of course, there's lots more, but that's off the top of my head. I don't see how anybody could consider Requiem for a Dream a popcorn flick, though..
