Having driven one the biggest difference is that the Volt is quieter and you don't hear the motor whine like you do in the Leaf. Driving position is lower. Seats are 10X more comfortable. Regen is more aggressive and accelerator is more responsive. Front seats have considerably more legroom. Back seats considerably less. Less headroom in back and front. Less overall cargo space. Cabin generally of much higher quality. Much less fade as you get towards 50 MPH.
Biggest surprise was accelerating hard from 60 MPH to 80 MPH. This is where the clutches come into play, and rather than being held back the acceleration was astonishingly smooth. I did it a couple of times just because it was shocking how smooth it was. The Leaf may be as smooth but it's not nearly as fast a transition so it doesn't give the same weird smooth feeling. No hard numbers but it felt substantially more efficient at 75 MPH, a speed which really takes a toll on the Leaf's range.