The stiffness & swingweight..combined with the mentality that players are going to strive for ultimate spin...anxious to see the physical effects on club players on arms, shoulders etc...
On another note, cannot wait to demo. Been using the Wilson BLX Tour 95(2012). Not sure if this 'S' stuff remains a gimmick or real game changer, even after TW's in depth reviews and playtesting. You would think if it was such a changer, pros would be flocking like they did for poly's, and all these testers would drop their current sticks for it.