It's as much as what they play as what they don't. I did it for years. It's a monkey's job, for sure, but you do know how to read a crowd and then have the tunes to back it up. The latter must be easier than ever. FWIW, I made a lot more money spinning CD's than I ever did in a paid band. For a while, I was snagging up to CAD$1000 a night, and this was in the 90's.