The force feedback transmission now has helical (diagonal) gears instead of spur (straight) gears. This significantly reduces the amount of noise and increases the smoothness of turning over the G25. There are, or should be, two different sites with reviews that I am hoping show the difference in sound side-by-side.
The pedals are largely unchanged except for two items. First, the brake and clutch have had the height of their standoffs increased by about 13mm. Second, there are additional threaded holes drilled in the pedal arms which allow the pedal faces to be moved left and right. Together these changes make heel and toe much easier.
The internals of the shifter have been tightened up and feel much better than the G25's. There is no sequential mode because the sequential mode hardware contributed to the way the G25 shifter felt in 6-speed mode.
The wheel works with PC, PS2, and PS3. All games that support the G25 should work with the G27 as well.
The shift indicator lights are already supported in a few games. GT5 Prologue is not one of them. It is impossible for me to say what other games might have support in the future because it is up to the game developer to announce support for hardware features, not the hardware manufacturer.
And finally, the wheel has four additional buttons on its face. This was obvious from the pictures, but it's a difference so there you go.