I think this question really means "What makes a diaper a REAL diaper" or "What makes a diaper a good diaper, or what a diaper should be." It's like when people ask "What makes a burger a burger?" or "What makes a Ferrari a Ferrari?" Sure, you can classify things as burgers or Ferraris, but some people will always think "Yeah, but those things are just burgers or Ferraris in name and in theory...but to me, they're not really a burger or Ferrari".
Because this is something that's different for everybody. I mean, sure, a cloth covered Pampers baby diaper of course is an actual diaper, but it means just about nothing to me.
So yeah...for me, it has to have a soft inside, a plastic cover, and has to make me feel babyish, or cuddly and cute.