On another level, however, I think we often forget that the "great" in the title is both relative and problematic. How do we define what is and is not a "great" SF book? When it comes to literature--or any creative project, for that matter--there are no hard-set definitions; there can't be precisely because to provide perfect, exception-less definitions is to imply that literature cannot change, that it is hopelessly standardized into a set group of features and objects. Science fiction can never be that. We've had the arguments over what "is" and "is not" science fiction before, here
and elsewhere, and those discussions rarely get anywhere.
So why the attempt to define "great?" In the end, the term will remain hopelessly relative. There is no point at which we can ever set "great" down and say "this is what great means for science fiction, and there are no relevant exceptions to it." What I consider to be "great" SF will likely run counter to another's view on the subject. Even if one agrees with my view of "great," there are bound to be varying degrees to that "great"-ness, to what one considers, as the author of io9's post suggests, to be an appropriate description or address of/to the "human condition." While I might agree with that, it doesn't explain what one means by "human condition," nor does it provide criteria one might say should go unspoken (the quality of the writing, for example, however relative that may be).
I think the questions should be: Does explaining what "great" SF is really matter? If we can agree that evaluative qualities such as those that would apply to "great" are relative and malleable, then shouldn't we wonder whether there is value in the term or in our opinions on "something?" How do we justify what is "great" in terms of its relativity, let alone the value of our opinions in a relative world?
I suppose where I'm going with this is here: If we can't say what is and is not "great," then can we as readers, reviewers, or what have you justify saying anything at all about the quality of a thing? I don't think there are any easy answers to that question. But I'll leave that up to you.