Since the release of this game, i am making a realistic handling for all vehicles and i can tell you exactly where are the biggest mistakes in the handling.dat file. : )
Unrealisticaly fast accerelation, too much grip (cornering and accer/brake grip), too much brake power, almost every vehicle have a large antirollbar force. These four are the most unrealistic.
They have also a value on how much to lose a traction at low speed, aka burnouts, and 4x4 jeeps have a bigger value (which means less grip) than cars, so a longer burnouts, which is a shame. : ) Also there are a material grip factor, which is too small, that's why vehicles almost do whatever they want on dirt and sand. Suspension levels are a bit stiffy also, especially the rebound damping ratio.
There are imposiblly small turning radiuses for all the vehicles and a too big steering angles.
And there are things that can't be tweaked from the handling file, like a countersteering aid and the slightly autobrake aid system, when you let of the gas.