Metal construction - better edge hold, smoother, more comfortable, capable of higher speeds, generally more damp.
Decambered nose/tail - a.k.a. early rise, a.k.a. tip rocker. A much better marriage of the curvatures of sidecut, nose kick, and camber. The nose slices instead of plows during a carve. Allows easier carve initiation and release, better terrain handling.
VSR - variable sidecut radius (optional). By using different radii in different sections of the board (nose, mid, tail), or even more complex mathematical curves, the board can be more versatile and more responsive to fore/aft weight shifts.
Those are the big ones. The improvements have been profound. You'll kick yourself you didn't upgrade sooner. Also the UPs were not race boards, they were freecarve boards. They were softer and turnier than their FP counterparts. Vague and mysterious marketing verbage in the catalog implied UPs were somehow higher performing than FPs, but that was just hype.