It has a terrible problem with "shadow banding." This is easily reproduced, and makes the camera's performance unacceptable. Nikon has once again screwed the loyal fanbois. I learned this by reading a couple different Internet forums. Of course, I haven't taken my camera out of the bag.
Seriously, though: has anybody who actually takes pictures like a sane photographer noticed this problem?
It has been demonstrated by boosting black portions of a frame 6+ stops and set off howls of it being unacceptable by the Canon trolls on DPR. But boosting 2 stops on their beloved Canon ruins the image entirely. The D7100 sensor is not as good in that respect as the D7000 or D800 but better than anything else...ever, from any company.
I'd never describe myself as a "sane photographer." People who know me might speak up. But I'll say this. The shadow noise on my D7000 is clearly superior to that on my D7100 when I look at my night football shots. However, the D7000 shots are often out of focus whereas the D7100 shots seldom are. Given the choice, I'll take in-focus shots with a little more shadow noise rather than cleaner out-of-focus shots.
I still haven't shot any athletic events with the 7100. But it doesn't seem to do any better with dogs in AF modes (so far) than the 7000. I didn't expect it to, honestly - the ability to discern where one malamute stops and the next one starts in an overlapped moving pack is asking a bit too much.
I have not seen the shadow banding issue. I do see shadows fall off faster, which I can use to my benefit in composition.
The only strange issue I've seen, and I've seen it only once, was something that looked more like chroma noise on the outline of a dog's fur, shot at iSO 800, and underexposed by about 1.5 - 2 stops. I haven't tried to reproduce the behavior, so I'm not sure what it was.
I've been learning video. The 7100 is much better for video than the 7000, IMO.
The D7100 files aren't quite as clean as the D7000's and there's a bit more colour noise, however the noise in general isn't unpleasant, more of a fine grain and the detail is held well. Pretty astounding for a 24MP APS-C sensor.
Now the D800 is something else entirely. Been living with mine for just over a week and it's still blowing my mind. Noise performance is a good stop ahead of the D7100 and the level of detail from those 36MP is astounding.
The two bodies compliment each other well though and the files have a similar look.
The AF system is one area where the D7100 outperforms the D7000. From my experience, it's faster, more accurate and keeps on focusing in very low light where the D7000 gives up. This was more than enough reason for me to upgrade.