Magnet, magnet wire. The rest is the ear of the beholder and a little consideration for utility. Remember when players yanked the "weak" PAFs out of their vintage Gibsons and replaced them with all manner of Duncan, Dimarzio, Bill Lawrence, and Mighty Mite pickups? Remember a decade or two later when you were then paying a grand or two for PAFs but could still get the "crappy" T-Tops for 30 or 40 bucks out of the repair guy's junk box? Now T-Tops are several hundred dollars. Remember when Fenders with Wide Range humbuckers were considered dogs, because nobody liked the pickups? They weren't the single coils that you wanted, and they didn't sound like Gibson humbuckers. Remember when Dearmond "hershey bars" and Teisco goldfoils were cheap-o crap that only economic bottom feeders would ever use? Remember the 80s when you looked at the Bill Lawrence pickups in the case at the local music store or in the Stew Mac Catalog and imagined all the guitars you'd put them in and how great they would sound, but now they're "inferior crap"? Those pickups all sound the same as they ever did. The public perception of their worthiness has changed.