It's insane how much talent there is in this thread. Really impressed with many of the posts. "World-class" is kind of subjective, but here are some of the things I'm best at.
I used to play Guitar Hero and Rock Band (as a guitarist) competitively. I wasn't world-class as far as getting 100% on crazy songs (but I definitely wasn't horrible; I could get about 750k on Through the Fire and Flames, if that gives you an idea), but I was definitely one of the best at calculating where to use your Star Power to get the highest score. On Rock Band 3, there was a point when I had the world record on about 40 songs on Expert Guitar.
I play Mario Kart Wii competitively. As far as the competitive community goes, I'm good but not elite. I've always been in the top American clans, but like, if you take the roster of the top American/Euro clan, I'd be slightly above average. (The Japanese community in Mario Kart Wii is pretty isolated from the rest of the world. The Japanese are a little better than the rest of the world, but not by a huge amount.) Excluding the Japanese, I'm in the top 100 people that still actively play, maybe top 50.
As far as non-gaming (aka, relevant, unless you're making money from gaming professionally, then I am jealous) accomplishments, I'm good at sports but definitely not good enough to even compete at a national level. In my second year of university, I got a 4.0 in seven third and fourth year math classes in a row (which means being in the top couple percent of the class), and in three of those seven classes, I got the highest grade in the class. I don't compete in math tournaments and I'm definitely not a math genius, but that's all I've got really and I hope it counts for something lol.
Man i never understood the optimal strats for guitar. Drumming in rock band was so much more easier, despite squeezes being a pile of shit. I used to have a few #1's in rock band 2. Considering i was pretty bad back then and sometimes the kits were unresponsive, i probably couldve done better if i stuck to it