The availability of suitable species for domestication always struck me as the most interesting part of his argument, as it's not one I'd seen before - but strangely it's the one that's not captured by the title "Guns, Germs & Steel" (which are, I suppose, the main proximate reasons why Europeans ended up decimating/overpowering populations elsewhere in the world, rather than more interesting fundamental reasons why Europeans got to the point they could travel the world possessing said items well before any other candidate civilizations did. Accepted, "germs" is related to ability to domesticate various animals, but hardly captures the full set of advantages that came from animal and crop domestication).
I never thought his explanations of why it couldn't have been China or India instead were that compelling though, not does he bother to explain why certain European nations became far more successful as colonial powers than others (e.g. why were Germany and Italy so late to the game, and unable to really compete with Spain/the Netherlands/Great Britain as far as colonisation efforts went).