I only have six things changed in GT5.
1.) More cars in races. 5 AIs doesn't bring out the intense racing experience. More like 20-25 cars would do good like Vision|Gran Turismo
2.) Realistic damage. Not too realistic or you're out of the race. More like Headlights, body, and tire explosion damage.
3.) Game physics. Make it seem like you're actually going 150 mph instead of making it look like you're going 60 mph.
4.) This will be shown in Vision|Gran Turismo but human physics. More action in the pit area.
5.) Real life day/night changes, weather.
6.) Better AI as well. More aggressive AIs and smarter driving skills. The AI in Tourist Trophy is pretty damn intense, but the 10 second penalty really kills you at the end. So lame.