In addition to a weather-sealed body, the D7100 offers some nice design improvements over the D7000.
Similar design to D7000
While there are a few control layout changes -- some of which I like, others of which I don't -- overall the D7100 has the same look and feel as the D7000. However, the new model is of sturdier construction; it's composed of magnesium alloy and weather-sealed much like the D300s.
There are four major changes here. Yay for the lock button on the mode dial, though I don't like it in the center. And there's now a built-in stereo microphone. Plus, Nikon's replaced the annoying dangly strap mounts with fixed rotating ones. But the movie record button is one of those too-small, poorly placed controls that's unfortunately become quite popular.
The real changes are on the back. The lock button is now integrated with the navigation control -- which I like -- but the record button has been moved to the top, where I don't like it. I liked the Live View toggle switch on the D7000, but the D7100 uses a more typical control. The i button on the bottom left is new and adds quick-menu functionality.