I find my D7000 to be more than a full stop better than my D90. To me the IQ and high ISO performance is a combination of low noise plus DR and detail in big crops. When you factor all of that in, my D7000 produces images that are equal to the D90 at more than 1.5 stops higher ISO.
If we get another 2/3 of a stop out of the D7100 over the D7000, I'll be elated. If we only get 1/3 stop of noise performance, I'll be equally happy, because the higher MP and better AF system will make the camera at least a stop more usable.
That said, there WILL be a learning curve with this new camera. The D7000 requires far better focus technique than the D90 to achieve sharp images. I expect something similar with the 24MP D7100.