I've often found routes in other games to be yawn-inducing in short order. Even some of the tracks in Hot Pursuit bored me at times. Dynamic weather also does a lot for the atmosphere. Driving in rain provides total immersion in the wet stuff. Not only do you have to deal with the drops pelting your windshield, but the water and mud whipped up by other drivers.
Sound effects are superb.
Engines whine and roar in convincing fashion. The audio also varies dramatically depending upon what you're currently trying to pull off. Leave the roadway and you're rewarded with the frightening sound of tires whirring helplessly in mid-air. Steve McQueen wannabes take note. Atmospheric effects are also very nice.