Is that really a qualitative difference though? Clearly, MP3 players had been around before the iPod. What set the iPod apart was much better design. This is, in a sense, a matter of quality checking, but to an extreme extent.

Does the extremity of the quality checking turn into something qualitatively different? That seems to be a rather subjective judgement, hence why you had the back-and-forth.

I don't think that is what set it apart. It was probably having a software program (iTunes), that could work with your hardware and manage your library. Not sure when they introduced the shop.