Sports and fast action shooting...ESPECIALLY if you shoot Raw. Switching from a 60D to a 7D, the biggest difference was the SPEED. While I frequently maxed the 60D's buffer during shooting, I never had to wait for the 7D. In fact, I found that I was taking so many more shots that I had to watch my memory card space!
However, I found high ISO/noise handling to be slightly worse (files required more processing to look good) and therefore a step back. The 60D was already noisy in low light as it is. That was disappointing and I hope for (much) better in the 7D2.