The new YZ450F is a complete package. There's no standout feature because everything works so well together. The new seat (which covers the fuel cap) allows the rider to sit wherever, especially on the very front of the seat. Unlike a lot of MX seats, there is great cushion. The motor, the suspension, the handling, the ergonomics...they all combine to make the new YZ a bike you can ride without thinking about it much.
The motor is much improved. Yamaha claims nearly 55 hp, but the standout characteristic is the underlying feeling of torque that the older YZ did not have. The new YZ is smoother on the bottom, yet it's reminiscent of a 250F in how quick it can rev. Outright power is impressive, and the excellent control via your wrist is just what a 450cc bike needs.