I just got Cho'gath, and jungled in a bot game to see how the route felt like. It went so well and so quickly I started wondering if I had been playing Nocturne wrong all this time. I didn't even rune up correctly for Cho; I used Noc's jungle setup, armor pen and all, and ended up selling three of my starting potions.
Then I tried again with Nocturne, same route, Vamp Scepter instead of cloth and pots, and it went much, much more smoothly than ever before.
So... I'm guessing it's the new masteries! I honestly didn't expect them to make that kind of difference, but going 21/9/0 instead of whatever it is I had before with the essential points in Utility for the extra exp seems to be a really big deal. I think the only thing I lose out on is the extra buff duration, which does hurt, but a faster and safer jungle might be worth it. Experience will tell, I suppose.