Re: What is a Classic?
I agree with both the phrases "enduring" and "breaks new ground" in terms of defining a classic.
To be truly great it also probably needs what Tolkien described as "applicability" which means it can re-interpreted from one generation to the next using terms unknown or unheard of in the author's time (e.g. Island of Dr Moreau & the idea of cloning/genetic experimentation).
Another way of defining a classic could be by saying it introduced new words to the human language.
An example would be "Cyberspace", which is generally attributed to Neuromancer by William Gibson (although I know this is contentious).
Classic could also be the start of a new genre or sub-genre in writing, I guess.