I've always identified very strongly with the apocalyptic/escapist lyrical subject matter of "Falling off the Edge of the World." Not so much about "closing the door" (I'm not that deep in despair; no worries), but rather the bit about "living well out of my time." I too should be at the table round, with Dio on my right and Launcelot on the left, ready to ride out in search of the Grail or rock the night with electric axe. Instead I'm a computer jockey at a desk all day. Oh well, one can dream.
On top of its wonderful thematic material, "Falling off the Edge of the World" features a great, deep, powerful riff by Tony Iommi, a great bassline, and of course Dio at the top of his game. A tough combo to beat. I'm also a sucker for the epic and that certainly describes "Falling off the Edge of the World" with its languid intro, morphing to a heavy drumbeat like the footsteps of an approaching doom, and finally switching to a high-gear eruption of sound at 2:07.
Turn it up, and enjoy.