Famous first words

"It was the best of times, it was the worst of times... "
"Midway in our life's journey, I went astray from the straight road and woke to find myself alone in a dark wood. "
"Not everybody knows how I killed old Phillip Mathers... "
"It was a bright cold day in April, and the clocks were striking thirteen. "
"As Gregor Samsa awoke one morning from uneasy dreams he found himself transformed in his bed into a gigantic insect. "

What makes a good opening? How do you convince people it's likely to be worth their while to keep on reading? Many of the great openings seem to be rather grim - or maybe that's just the books I read.

I've heard that you should 'start a bomb ticking' early on, or revise your opening to make sure you start 'as the kettle comes to the boil'. Personally I've always liked the idea of starting with the death of the main protagonist. (I just need to get the rest of the plot worked out... )