Of the bands that I have not seen, I think the ones I'm most itching to see are:

Rage Against the Machine (they could still get back together some day... right?)
Tenacious D
Marilyn Manson (Only if he gets his act together and isn't a total disaster. I wish I would have caught him a decade ago.)
Eminem
How to Destroy Angels
Guns N' Roses
Rob Zombie (I was more of a fan of White Zombie, but I've heard he puts on a fun show.)
Aerosmith
Red Hot Chili Peppers
The Eagles (I bought my parents tickets for their x-mas present, but I wasn't willing to pay another $150 for my own ticket)