Great physics (apart from some cars that require tinkering with their default setups) combined with vast improvements to FFB on steering wheel, sense of speed, cameras, immersion, and some other things over FM7 are enough for me to keep playing this game since launch and enjoy it more than any previous Forza, despite so many shortcomings.
I like the car roster too (despite all those missing models), it’s large and diverse enough to keep me discovering new and exciting stuff for hundreds of hours.
The track mastery system with section scoring is another feature I adore, I wasn’t so invested in driving a lap after lap since at least Shift 2: Unleashed.
Also graphics looks very pretty (mostly), although I needed to swap my RTX 2080 for 4080 and start playing in 4K to really appreciate lighting, reflections, dynamic time of day, environment details, etc. I wish ForzaVista wasn’t so downgraded so that I could fully appreciate details of new car models as well 
And the career mode, while shallow and very light on content, keeps me entertained thanks to diverse events with cool themes and introductions. If only they weren’t removed from the game after few weeks…