My impression from several hours of the game now:
1. Career - it's damn awesome, easily the best career of any FM game, what I like is that it uses available ribbons progressively, so eventually I assume you'll drive all of them, this is great, Forza 5 got stale real quick because it didn't even make full use of it's available tracks. The career doesn't feel like a grind, but a steady progression.
2. Rivals mode - something that I haven't used much in FM4 and 5 because it was kind of convoluted and didn't make much sense to me. In FM6 I might actually put some serious time in it, the car is provided (love this), you can clearly see the time that you have to beat, you can clearly see Worldwide/regional/friends leaderboard right there above the rivals event. Just seems better realized to me.
3. AI - it's fantastic, once I've landed on the right difficulty setting for me I've had some really tight races, going three cars wide into a corner and not colliding with each other, jesus, I can't remember a single time that happened to me in any other game. T10 has certainly made some adjustments in their skill and behavior, less pushing around and more cool moves and techniques - saw one drivatar consistently fish tail around corners without losing control, looked very impressive. Overall my races felt tight, but pretty clean, again I'm impressed.
4. Track list - this is probably the best thing in the game, so many great tracks that I've wanted to see in Forza for ages, it's very robust and diverse. The only way to make it better is to bring japanese tracks back. Plus all of them look beautiful, easily the best looking tracks in a simracer imo. Nice details like dust and mist add to the atmosphere.
5. Car list - again very diverse and a lot of fun cars I'm eager to drive.
6. Physics - is a massive improvement over Forza 5, no contest, the cars feel a lot more grippy and planted, the edge of traction is well communicated through the controller and overall I'm having a lot more fun, whilst in FM5 I had to fight with sliding around way too much.
That's all for now from me.