One of my biggest grievances about rap (and I have many) is the age-old practice of "sampling" - basically, taking part of someone else's song and sticking it in your own. It can be relatively small and benign, or be used to build a great song (Cantaloop by Us3, or a lot of the brilliant stuff that Public Enemy has done), or it can be blatant and really suck ass (most of MC Hammer's body of work). The problem is when it becomes endemic to a genre, and leads to the sacrifice of creativity for the sake of familiarity.
Now, I know that sampling and covering are also done in other areas of music, including rock (right now I'm in a coffee shop, listening to an American Idol wannabe do an overwrought version of Jeff Buckley doing Leonard Cohen's "Hallelujah"). But I've always considered rap to be the biggest offender. It wasn't until I started to listen to KPLU, the jazz station in Tacoma, on a regular basis that I realized how much borrowing goes on in jazz as well. I'm not just talking about Kenny G's Christmas album (the impetus for one of the greatest lines by Norm Macdonald, or anyone for that matter, of all time: "Happy birthday, Baby Jesus! I hope you like crap!"). No, no - it seems like every other song being played was done by someone else before. Whether it's Vijay Iyer playing Michael Jackson, or Denise Donatelli doing Sting, it usually strikes you the same way: you listen for a few seconds, start to get excited, and then realize why that melody sounds so familiar.
Of course, jazzmen and women do it a little more skillfully than most hack-ass scratch-rappin' DJ's, and some would say that re-worked covers have had a vital role in the history of jazz. Think about what John Coltrane did with "My Favorite Things," or Miles Davis with "Porgy and Bess." And hell, I myself have covered songs by Roy Orbison and the Blue Orchids on a recording. But still ... you know? But still. What would jazz be like if jazzmen (and women) weren't so busy playing stuff that had already been done by someone else? Not *quite* so busy? I bet it'd be pretty cool.