I would say the realism is in the ability to change each car's handling with the use of settings adjustment. I have never played another console game with such an indepth system to simulate each element of a suspension's effects on the handling of a car.
Next would be the extensive list of real world road and race cars. Enough said!
After that would be the track setups... each track has amazing surface design. Some places dip in softly and others are rough on the fringe. Just go to trail mtn, and race something nice like a 787, then go place "nfs hp2" on any track of choice... I garuntee nfs has half the depth of track surfacing (saying that out of experience).
The way each track's characteristics change a car's handling is also unmatched in most games.
Another part of the realism and simulation is the braking and recognition of distance. Go ahead and bash it if you want, but before gran turismo the world of console racing was sorely missing some realism with regard to cornering speed and braking distance.
Now, I have to ask...
What view do you use?
If you're not in first person, try using 1st person view.
Then use a fast car and try getting as fast as possible on something like Tokyo 246 (and of course, running clean laps).
If that doesn't change your opinion then I guess gt isn't the game for you.
Also, don't forget that gt tries to keep the "world" in focus, and because of that you wouln't get special blurring of vision at high speed (atleast, not in gt3... would be nice in gt4 though).
So yeah,
Hope that gives you the other perspective.
