The ASM Hydrasynth is interesting for several reasons.
Firstly, and most obviously, its keyboard has polyphonic aftertouch. This is something players have been yearning for ever since the Yamaha CS-80. It has always been considered too expensive to implement, but somehow ASM have done it for a very reasonable price.
Secondly, it has a keyboard-wide ribbon controller; again, something that the CS-80 had but we have not seen much of since.
One of the features I find most interesting is that fact that the envelope components (attack, decay, release) may all be synced to MIDI tempo. This means that we could set attack to be zero, decay to be a quarter note and loop the envelope.