What makes an artist classic?

This doesn't have anything to do with anything - just a quick thought in my brain to kill some time as I eat my tasty Taco Bell Spicy Chicken Enchilada Platter (with a large Baja Blast to boot).

Last night, in my Modern Continental Literature class, we got to talking. What makes an artist (define that term however you see fit) a "classic"? The examples I brought up were from what we consider "classic" rock - the Beatles, the Stones, Aerosmith, and so on. The professor asked if there were any musicians from my generation that I thought would still be remembered in thirty years, and maybe even considered classic.

Here are three artists from "my generation" (whatever that means) that I think will be considered classic in their own right, and a YouTube link for each if you need further convincing.

1) Nirvana. These guys redefined the idea of rock with four simple chords. "Smells Like Teen Spirit" is an iconic masterpiece that shaped the face of modern rock. http://www.youtube.com/watch?v=kPQR-OsH0RQ

2) The Smashing Pumpkins. "Mellon Collie and the Infinite Sadness" is one of the best albums of the last 30 years. It's epic. Go buy it and thank me.
http://www.youtube.com/watch?v=EsZYqaSc4cU

3) Pantera. As a metalhead, I have to include these guys. If Dimebag Darrell hadn't been shot in 2004, I really think we'd be seeing a reunion of this amazing band.
http://www.youtube.com/watch?v=87uEuhMj92k

All for now. Got a sermon to write for Sunday.

~Sean~