The talent to pull it off?
Okay, really, for me, and I agree with Kambei that something like this is totally in the eye of the beholder and can be very personal, but for me, it's the ability to turn out a custom that can stand side-by-side with production figures from Hasbro, and you can show a line-up of figures to someone that maybe has SOME knowledge of G.I. Joe but doesn't know every character, and he doesn't realize that there's a couple of customs in there. That's my primary objective -- to create a figure that "blends" as well as possible. Or certainly, is on a par with either creating an interesting new character, or coming up with a really effective version of an established character from other Joe media that just didn't get made as an action figure.