Obviously the AI drivers are more skilled and the tracks are more difficult, but is there anything else? I ask because I swear the handling of my cars has changed. (I play with all aides on as I have a wheel but no pedals.)
I should also add that I'm not at all familiar with rallying.
Along with the things said, your choices for cars opens up a little further, and the upgrades you have to apply to your cars for pro races make them faster. The upgrade to all-star will also cost you to upgrade your existing cars and they will be faster again.
Sure the grip is different! If you want to win Pro, you will possibly make adjustments to the basic settings. In addition, each victory or podium will give you more money and reputation point (level up faster).