I've just watched a thorough walkthrough of the SZ and there are two things which i think could and should be implemented onto the SX.
Needle Search. With the SZ, Needle Search is disabled when a track is playing; it is only operable once the track on that deck is paused / stopped. This negates any accidental touching of the strip bar when you're playing with the FX.
Secondary function on the "Hot Cues, Roll, Slicer. Sampler" buttons. More specifically, i would like to see the Hot Loops (secondary function of Hot Cue button) to be implemented onto the SX.
*FEEL FREE TO ADD ANY MORE*
I don't believe it'll be that difficult for Pioneer to release a new firmware update for the SX to include the above features.
However, do let me know if anyone's been able to acquire the above two functions on their SX through some black magic (mapping it yourself).
Cheers!