There are a LOT of differences. Hard drive, more polyphony, more styles, more sounds, larger display, more provisions for editing, more outputs, more pedal commections, aftertouch, INDIVIDUAL BUTTONS FOR ALL ENDINGS AND INTROS, quicker access to more registrations; these all immediately come to mind. A major feature is that the operating system is in RAM so can be updated by software. The key feel is more solid.
I thought the Tyros drums sounded better than the 2100 also. I'm sure there is much more. I only played one for a couple of hours.
DonM