The frame rate is probably the main thing keeping me from upgrading from my D200. My primary need for a faster frame rate is equestrian jumping. I'm shooting younger riders going over jumps about 3 feet high. At this height the horse is 'airborne' for only a very brief time and there is a sweet spot at the height of the jump where the horse's front legs a flexed in tightly and the rider is leaning towards the horse's neck. This is the shot the riders tend to like most. My D200 has a frame rate of 5. At that frame rate I'm batting about 500 on getting that perfect shot. Certainly timing and reflexes on my part are important but every horse and rider approach and jump differently. So in my mind, the higher the frame rate the higher my chances of success. The D7100 has a rate of six so it would be a step up from the D200.
A big enough step up? I don't know. I'll be checking this thread often to see what the consensus is.