My favorite bands to have seen "original and complete" would have to be:
Either because they broke up and then someone up and died, or because they will never get back together and I'll never have that live experience, no matter how many old films I watch.
Then there are artists like Nick Drake and Dan Fogelberg who aren't "bands" - just solo performers I never saw and would have loved to :-|
What about you? Which bands do you regret either being too young to see live or just missed your chance and kick yourself now about it?