I've spent a lot of time talking about art with art friends. At this point, we've all seem to agree on three different aspects of art:

1. Non-random uniqueness / creativity. Even is something is lame, originality alone can constitute art. (e.g. weird, uncomfortable stuff)

2. Story. The context or meaning behind a thing can take something done before and make it new again. (e.g I made a rubber band ball for my ex-lover...)

3. Skill. Something can be totally derivative and bland, but if it's really hard to do.. the act of doing becomes the art. (e.g. high-quality muzak)

One thing not in this list is "familiarity." Even though familiarity is most humans favorite thing. Familiarity is actually a major aspect of what defines popular media.

I find that mash-ups lean really hard on the positive connotations of familiarity.