I tend to agree with
@Aldarc that the Law/Chaos axis can provide a more nuanced and morally complex conflict than the Good/Evil one, but I'm not - on balance (forgive the pun) - against the inclusion of Good and Evil as fundamental elements in the D&D game.
D&D deals primarily with caricature and cliché and does not weather the postmodern critique terribly well. Evil is Evil because it is Evil; it is the evil of fairy tales (the evil witch; the evil troll; the evil demon), not the evil of Adolf Eichmann or Jeffrey Dahmer - or, rather D&D lacks the mechanisms to adequately deal with (actual, human) evil and deals instead with the idealized; the fantastic. It is the evil which is vile and blasphemous; it profanes and violates and desecrates; it is an abomination.
Likewise, Good is Good because it is Good. It's the good of angels and good fairies and chivalric ideals. It is sacred, and holy; it sanctifies and consecrates and heals.
If these definitions seem tautologous, well, that's kind of my point. And I'm not saying that you can't have fun challenging, subverting, inverting and generally messing with these expectations - far from it. But I think if you cut out good and evil, you are also excising some of the mythic power of D&D to tell certain stories. Stories which are - necessarily - rooted in a particular (Westernized/Christianized) vision of the world, but powerful stories nonetheless.