As a companion to the Top Five Bands You Want To See Live thread, let's do one for top five bands who are no longer alive or actively performing that you which you could have seen live! Here are mine:
1. Nirvana
2. Jeff Buckley
3. Pink Floyd
4. Led Zeppelin
5. The Doors
And then there are bands like Guns N' Roses, KMFDM, etc., who are still around, but I wish I had seen them in their heyday.
Also, I was lucky enough for my first show ever to be NIN & Bowie, but I didn't fully appreciate Bowie yet. I would have loved to see him again after really falling in love with his music.