Ok, just got this (despite some difficulties using the Xbone store...) and here are some super duper preliminary impressions.
Graphics are fantastic, except there seem to be a lot of shimmery jaggies on some of the lines of the cars. It's less noticeable if the camera stays still, but as the camera pans around the little white jaggies flicker and become more prominent
Boku bonus points for having the Subaru BRZ available as a starter car. I'm a big Subaru fanboy and just recently I was thinking of checking out the BRZ at a dealership just to try and wiggle my way into a test drive.
Drivatar concept is fantastic. It was such a trip in my first race to see GTers names as my opponents, and as I would have expected in a live match I was passing up people I would generally beat in live multiplayer and ended up with me and Gellar battling for 1st. Too early for me to tell for sure if it's imitating driving styles well, but at the very least it was nice to have my AI opponents not all driving the same line identically.
Music sucks so far. Epic instrumental/chorale music isn't want I want in a racing game. Hoping something more rock or pop shows up, but I usually turn off the music in these games anyhow.
Cockpit view looks great.
Gamepad controls are awesome.
The Forza games have always been one of the better racing games for controller use, but the trigger force feedback seems really really well done. That said, I still wish I could plug in my 360 steering wheel.