I don't want to offend you, but if you're used to Gran Turismo 4, you've become accustomed to a physics engine that greatly underemphasizes the oversteer abilities of cars. When a car is near the limit in real life, it actually doesn't take that much work to bring the tail end out (of course this depends on the car in question and the way its suspension is tuned).
I will admit that I have yet to play GT5 Prologue myself, but from what I've seen and heard, particularly from GTPlanet members I consider trustworthy, it's taken nothing but steps in the right direction from GT4.