Playing some more, I feel like I've figured out what the problem is, at least for me - it feels like too much horizontal momentum gets converted to vertical momentum when you crash into anything remotely resembling a ramp or hill. I remember a long time ago Chris (or someone on the dev team) said that they wanted GRIP cars to "slam to a halt" when you ran into a wall, as specifically opposed to how you get launched in Rollcage. Which happens... if you hit a perfectly vertical wall dead-on from a flat surface. As soon as uneven terrain gets put into the equation, the cars turn into Saturn V's and go flying upwards.
Also, I agree with Demigan's points. Even without being on "Wild" engine settings, collisions (particularly with other vehicles) seem too ready to throw you completely off-kilter and causing you to crash. Citing the same example he did, if I'm going 450 km/h and a car in front of me is going around 300 km/h, I should blast him out of the way when I rear-end him, rather than him getting sped-up and me spinning out of control.
Also I really agree with what he is saying about physics feeling the same in any direction, which is probably a much better explanation of my earlier complaint (in a different thread) that sliding sideways feeling like I was on ice. It's because sliding sideways is treated the same (or too similar to) rolling forward or backwards. There's no (or very little) additional friction or traction when your wheels are perpendicular to the direction of motion, which causes slides and losses of control to feel extremely unnatural.