The main improvements of the PSR2100 are 1) 20 extra onboard styles, 2) some new voices, 3) fade-out button.
Of the new voices, the piano is the main improvement. There are a couple of cool organ voices and a sweet harmonica voice that you will like. None of the 160 or so old styles that were on the PSR2000 are revoiced on the PSR2100, so if you want to take advantage of the new voices within the PSR2100's styles, you have to revoice the styles manually and save them as registrations.
Overall, the PSR2000 and PSR2100 are almost identical. When I moved up from the PSR740 to the PSR2000, my excitement lasted a couple of months. When I moved up from the PSR2000 to the PSR2100, my excitement lasted about two days. When I moved up from the PSR2000 to the PSR3000, my excitement lasted a couple of months again.
I also highly recommend moving up to the PSR2000, PSR2100, or if you have the bucks, the PSR3000. You will have a lot more fun.
Beakybird