Here are my thoughts:
-The physics are better, if only slightly. Not as arcade I suppose, I like them.
-The A.I. have no earthly idea that you exist, so be forewarned. They will hit you. Hard.
-The photo mode is better, I think. I am no expert on the matter. But I hate having to upload my photos to another server. My crappy internet never connects, so I quit trying.
-Car sounds seem to be better as well. I quite like the noise the SLS makes. I am also amused by the lack of noise my Volt makes, forcing me to watch the RPM gauge the entire time I drive so I know when to shift.
-Making money is hilariously easy. So easy that there is no reason to cheat for money this time around. I actually don't think they could have made it any easier, save for handing you money every time you turn on the game. Oh wait, they do.
-If making money wasn't easy enough, if you drive a car few times, you eventually get the performance upgrades for free, giving you no reason to spend the hilarious amounts of money you have acquired.
-You can turn the class restrictions off, which makes for a lolafiric time. Just grab a fast car, enter an F class race with the A.I. difficulty set to "professional", and watch the credits roll in.
-The auction house is a wonderful place to make money. I don't want most of the cars they offer me for leveling up, so I always put them up for auction. For some bizarre reason, other players consistently bid my vehicles to prices that are higher than their in-game value. I'm not complaining, simply confused.
-Cars are even easy to get. Almost any car a person could desire care given to you at some point, giving you even less of a reason to spend your hilarious amounts of money. Zonda Cinque? Given to you. McLaren F1? Given to you. Bugati Veyron Super Sports? Given to you. The list goes on.
Overall, I would say the game is more fun than FM3, but we will see how long that lasts.