I noticed some peculiarities with Forza 3:
FIA curbs slow you down, and grass just bogs down your car. I guess they had to figure out a way to penalize you for cutting a bit of the corners, but last I checked rumble strips shouldn't slow you down that much...
They also used different layouts for the different tracks like Catalunya. The full grand prix course felt weird to me. I don't remember Turns 10-11 and Turns 14-15 from GTR2, GTR Evo, and MotoGP... I know they use a slightly different layout depending on the race class but I only saw the "Grand Prix" Catalunya layout for Forza3 being the full course.
Lift-off oversteer is almost non-existent on the lower tier FWD cars, I might just have to experiment a little more?
I noticed they have the Cobalt SS Supercharged but not the Cobalt SS Turbocharged. They have the Solstice, but not the GXP. They have the Saturn Sky Redline which is a C-class car - but it has the same weight as the Solstice and Cobalt SS turbocharged, with the same LNF turbo engine as the GXP and SS/Turbocharged. It would have been nice to see the Cobalt SS/TC in there as a C-class car that cost only $20k USD new. But they managed to plop in the $42k+ Taurus SHO as a D-class car...