I love GT, having played each one extensively from the original up to GT5P, and the one thing that really grinds my gears is the constant tyre squeal. No matter what compound tyre you have selected for the race, as soon as you even think about turning the wheel, the tyres start to squeal. Not only is it completely unrealistic (this is supposed to be a "The Real Driving Simulator") it's also very irritating too.
The lack of tyre smoke and laid down rubber are also bug bears of mine, but nowhere near as much as that god damn constant squealing.
That said though, I'll be getting the game on release day regardless.