Put it simple – the D7100 is an improved D7000.
The D7100 now has 24MP (instead of 16MP) sensor, with the optical low-pass filter removed. It has 51 AF points (15 crosstype) instead of D7000′s 39 AF point (9 cross-type). The D7100 also now comes with 1080 60i / 30p video, unlike 1080/24p for D7000.
The D7100 is said to also inherit D4-derived algorithms, making it more versatile in capturing fast action (eg sports). The better processing should offer better color reproduction and improved noise reduction, much like the D4.
The built-in microphones is now stereo, and it has an optional 1.3X crop mode, giving an effective focal length increase of 2X (making a 50mm lens behave like a 100mm). How this works is that it multiplies the already 1.5x crop factor of the APS-C sensor by 1.3x. So the field of view you get in the crop factor mode, is nearly doubled.
So supposedly you’re using a 200mm lens. Now multiply that by 1.5x for APS-C crop factor and you get 300mm. After that, multiply it again by 1.3x so it’s now 390mm!
In cropped mode, the still photos are reduced to 15MP with burst rate going to 7fps from the original 6fps.