I added my point.
Gt5 and forza, if played with the correct minded people, enhances driving etiquette when on the race track. You enhance your skills around others with such close racing and you allow yourself to become more comfortable knowing the boundaries of pressuring someone and keeping someone from passing. There are awesome points to be made, but as far as online play is concerned, I'd say that's the biggest!
I'm not so sure if I have learned anything apart from transferring my skills of drifting/sliding cars is, how do I put this? Ridiculously difficult to pick up in gran turismo. It's the one thing I'm struggling with on my dfgt! I can race, and pretty well at thy but the sideways part, which many people praise the game for, is a bit off for me!