Bands/artists you never got to see live in their heyday, but would've liked to.

Genesis on The Lamb Lies Down on Broadway tour (Gabriel's final tour with the band). My best friend got sick, so I missed the show.

Renaissance and Spirit. I missed the show because I got sick. 

Badfinger on my 18th birthday, had to be 21 to get in. State lowered the drinking age to 18 the next year....a year too late for me.

J S Bach

Chopin

The Who (but my hearing thanks me. For a while they were loudest band in the world)

Pink Floyd

Talking Heads mentioned a lot here, but I did see them during their 'Stop Making Sense' tour, and David Byrne has to be the most entertaining front man I ever saw.

Also saw the original King Crimson line-up in 1971, another terrific show.

What a great thread!

1968-69 Beatles 

Allman Brothers, with Duane.

Bill Evans

Diana Krall

Dire Straits

Ella

Genesis with Peter Gabriel

Genesis after PG, with Phil Collins

and many many more!