There are a handful of GC games that haven't come to the states worth playing... but not much.
Nintendo Puzzle Collection - I play this all the time with my girlfriend, tho the games are all emulatable at least 2 player on the Xbox. It does feature 4 player Panel de Pon tho (Tetris Attack,) so it's nice for that much.
Mr Driller Drill Land - A kickass compilation of different types of Mr. Driller. The music rocks in this, and has some great 4 player modes to boot.
Doshin the Giant - Obscure Populous-style game, playing a big odd giant, and getting the humans below to worship you. Worth looking into, at least.
Past that, it's mostly just getting the jump on releases that's appealing. I dug playing Zelda - 4 Swords, Final Fantasy CC and Wario Ware nearly a half year before domestic release.
Overall, I think import gaming is well enough covered by the easy switch installation and swap disc option. I'm mostly psyched by the idea of possible homebrew creations. While the Xbox covered most bases needed, it might be nice if someone could hijack the N64 emulator used for the recent Zelda 64 rereleases.