I must agree, the physics in Halo Reach were way out of whack. On the other hand, like many people have already said, Halo 3 physics were the best. And I think 343 Industries is going to address this in our favors. It seems that 343 is all about realism. I mean, they made an actual story about why Red Spartans fight Blue Spartan just so there would be some sort of realism. They also created a new breed of Spartans for multiplayer to help convey the story more effectively.
So all signs seem to point to the fact that 343 Industries is all about realism, or in this case, getting Halo as real as it can be without ******** us fans off. So I think the physics of a Warthog will surly by fixed because, even in the year 2557, there is no reason why a Warthog should fly 100 feet into the air when a grenade hits it. If a grenade was truly that powerful, the Warthog should look like it had been nuked by the time it hits the ground.
Just my two-cents, hopefully 343 really does follow through like I expect them to.