Everything! For my needs, the 5D3 is about as close to perfection as you can get. The world-class AF system, more-than-adequate burst rate, brilliant LCD, idiot-proof menu system, dual card slots, customizable buttons, squeaky clean high ISO performance, ridiculously good battery life, and rock solid build quality make it the best dollar-per-dollar camera on the market, even at $3,500.
The only changes I'd make to it are either dual CF slots, the same CF/SD arrangement with a faster SD slot, or a larger buffer. With a 90 mb/s CF card, I can fire off 23 RAW images before the buffer fills up, which is more than plenty. However, plugging in the SD card reduces that figure to 10 RAW images before the buffer fills. Recording RAW to the CF and jpeg to the SD slows it down even more. The AF coverage is outstanding, but the greedy photog in me wouldn't mind if the AF points covered an even larger percentage of the frame.
Overall, I'd say that Canon knocked it out of the park with the 5D3, and it's made my shoots much more enjoyable. In contrast, while I loved my 5DC's image quality, I hated using the damn thing.