Redefining gaming comes in two forms, what the software does and what the hardware does.
Taking gameplay:
16 cars instead of 8 enchances gameplay
Weather / Night Day races enchances gameplay
Car modifiying / customisation enchances gameplay
Online mdes enchances gameplay
Taking hardware: (Below Not Possible on GT4 PS2)
Broadband enchances online gaming features
HD 1080p visuals enchances graphics
7.1 PCM enchances audio
3D enchances immersion
Headtracking enchances interaction and simulation
Steering wheel with clutch and H shifter enchances interaction and simulation
Headtracking was listed on the official GT Website thats good enough confirmation for me so dont argue over it as all the other features listed on that update are also assumed confirmed and expected in the game.
Feature List
Sept 9th 2009
The point being made is GT5 looks to be very much a genre defining possible title...
If they put this effort into the hardware then we should assume the game goes a bit further in gameplay/simulation/racing modes as well.