Personally, my problem is lack of originality. You give me existing rule systems, art, and/or settings, and I can learn them and expand their boundaries without breaking a sweat. However, if I try to draw my own art, or create my own mechanics, they just end up looking like crap, or being far,
far too complicated.
This is sort of the reason the only games I've ever run were originally started by someone else, with established mechanics and stuff.
CTF: a reboot of the version Notimportant used to run, which used his base rule set. The setting was basically that of a distant sequel, and even though I didn't use his art directly, I did base everything off Moose's work on it. So there was practically nothing original.
Pokemon: taken from Burns after she couldn't continue it, I'm using an expanded, modified version of her original plot, and mechanics taken straight from the interwebs. It's mostly text, but even the little art it
does have (battle maps), I'm in the process of shamelessly stealing from Donnigan's game
So yeah, I can fix/improve stuff, but not create my own without making something hideously complicated and ugly