Having now put a couple (hundred) hours into it I have some additional thoughts on the game:
A: Best soundtrack in a Forza game to date. Motorsport or Horizon. In all the previous games I've ended up turning the music of completely very quickly. Menus, in game, in races, everywhere. Not so here.
B: Kiera and Warren are incredibly likeable characters. Which is weird, but their scripts and voices both are funny and something you don't mind hearing. Kudos to the voice actor/actress.
C: Car selection is a bit odd, but bigger and better than any of the previous Horizon games. The lack of VW is felt rather strongly by me, I used the 1988 Scirocco constantly. But at least I have another go-to: Volvo 850 R. My first car. Which is fun as shit.
D: Showcases are still annoying. And still rubber banded to hell.
E: Being the boss of the show is fun and works well. The blueprints and stuff add a lot to the game (IMO).
F: Australia was a great choice for location. The beauty spots and danger signs are gorgeous and fun, respectively.
Downside: After the last patch (which fixed one of the major issues I had) I am still getting random crashes to desktop. Playing on Xbox One means I get no error message when it does so. This means I have no way to report it beyond it happening.
I have yet to beat the game. I have yet to get even close. I have over 120 cars so far. The timers in the menus don't agree with each other. Even the total miles driven don't jive. My Volvo has almost 2,000 miles on it. The game itself says I have only done 1,200 miles. I had the same problem in both previous Horizon games so I'm not totally surprised.
Final score: 9
I don't know how they'll top this in two years. Chances are good though that I'll still be playing this by then. The location is amazing. The music selection is great. Car list works well and is varied enough that you have a car for all terrains contained in the game. The buggies rock by the way.