At the Yamaha booth where the PSR-2100 was being shown, the information about it stated that it had the PSR-9000 sounds plus 50%. What that meant who knows but I took it to mean that it would have 48MB of ROM (32MB from PSR9000 + 16MB more (50% more), which is exactly the same as the 9000 Pro. Scrolling through the sound banks it appeared that might just be the case.
The keys of the PSR-2100 while different feeling from the PSR-2000, by no means where they great feeling but they were better.(definitely not the same as the Motif though).
The build quality of the PSR-2100 is identical to the PSR-2000 (same chassis) and it reeks of cheap. For a home setup its fine but if you plan on gigging with it heavily, its not going to hold up over time. Then again that's not what the PSR-2100 (and PSR-2000) were designed for so using them in that fashion is at your own risk.
I have to credit Yamaha for adding so many new sounds, a USB port, and extra flash RAM and keeping the price the same as the PSR-2000. Clearly the PSR-2100 is going to be a huge hit for Yamaha. The chance of finding another arranger keyboard in the same price range with so many great sounds will be quite difficult if not near impossible.