I'm enjoying this discussion, and getting a lot of insight from it. Ddw, I think the blues is a varied musical form, and like most music it can't be summed up completely by one or two "signature" characteristics. And I will grant you that there are blues that don't seem to emphasize, or even include, all of the defining characteristics. But you have to start somewhere, and for me the "minor against a major" tonality (as embodied in a minor third played against a major I-IV-V chord progression) is a good place to start. As M.Ted correctly points out, there are other "tasty intervals" (well said!) that also are characteristic of the blues, and can give other musics a "bluesy" quality. There's also the standard 12-bar structure, although I think most of us recognize that the 12-bar form isn't an absolute requirement by any means, nor is it limited to the blues. And I agree with you that the slow tempo is probably not something that should be seen as an essential element of the blues.

As for relative minors, that's really another matter. Western musics have made use of relative minors (the Am in the key of C, for example) as a basic building block for hundreds of years; this should not be seen as a defining characteristic of the blues. The blues took certain tonalities that would be considered "wrong" in conventional western music, and employed them to great effect. Then we all took that basic start and ran with it, adding in other elements that broadened the music -- a good thing, but it makes it harder for all of us to settle on a concise definition. That's why we're having so much fun now...