I've owned the D7000 for about two years and i Pre-Ordered the D7100 the day it was announced..
I shoot high school and pee-wee sports (field and indoor sports) and while i LOVED my D7000 i think its worth the few hundred $$ i'll loose by 'upgrading' to the D7100.
Here is my list of things i'm looking forward to as an ex D7000 owner (sold my D7000 this week)..
1) The new High intensity / High-contrast display in the viewfinder! I don't always get to pick what sidelines i shoot from (If you've ever shot HS varsity Rugby you'll know its sometimes not safe on the other teams side ). In bright daylight its sometimes hard for me to see the display in the viewfinder on the D7000. Maybe its because im getting old, but there have been times I've shot a quarter with an f/stop wrong because i couldn't see the display due to sun-glare / brightness. (i never said i was a pro!)
2) The Virtual horizon is moved off the exposure bar and onto the viewfinder display. On the D7000 you held down the FN button and the Virtual horizon would replace the exposure information in the viewfinder. Now (hopefully) its on the display all the time.
3) The AE-L/AF-L button has been moved over just a smidgen. While most people don't care or even notice... i prefer 'back button autofocus' and im tired of my finger sticking in my eye while i hold down the AE-L/AF-L button because NIKON WONT GOVE ME A DEDICATED AF-ON button! (I don't believe in the D400... so don't tell me to wait for it). Moving over the AE-L/AF-L even just a hair will help!
4) D800/D300s level weather sealing! It's hard enough for me not to get wet at youth swim meets.. asking the camera to stay dry is even harder.
5) Mode lock button. I use the U1 and U2 settings all the time.. mostly so i can set the camera back to 'auto' if i have to hand it to my wife. There have been times she's pulled my camera out of the bag only to have the mode dial move on her.. then she gets mad at me because she couldn't get the camera to work (see my comment about back-button-autofocus).
6) The grip is wider then the D7000.
7) There are a few more Mega Pixels.
8) 51 point autofocus with "an algorithm equivalent to that of the D4"
9) 1.3x crop mode might be interesting.
But mostly its just a few little things that have me the most excited
And heck.. with all the D800 and D600 problems you know Nikon is making every single D7100 perfect at launch right?!?! There should be no problems being an early adopter!!