The physics for all the individual cars in the game have been revamped. Every car now has its own controls, and ways in which it will react to your driving.
With the new physics model, it is possible to pull of insane tricks. The physics model is as close as possible to real-life, while still keeping the fun in it. You will be able to suddenly turn you car at a high speed to make it tilt on two wheels. Just using your imagination will be fun!
There is a large variety of tricks which are able to be pulled of on the motorcycles. Just like in read life you will be able to tilt your weight in different directions to get a reaction out of the bike. This will provide for wheelies and just about anything else you can think of.
Boats provide a huge new way of transportation, as in GTA3 they were very limited. All the boat physics have been redesigned to make them more fluid, and weather will greatly affect your ability to steer these nautical masterpieces.