The good:
The best graphics (some effects are straight beautiful; I crashed once because I was too busy admiring the sunset and lighting lol)
Awesome weather transitions
Still the same tried and true Forza physics for the most part
Homologation is fantastic (albeit a bit confusing at first) in some divisions (especially Forza GT)
You can tune multiple cars without having to load the track again.
Tracks now load in the background while players are still able to use lobby menus.
Little wiggles of wipers and wings are a nice touch.
The game plays great for the most part.
Auction.
Driver gear? (can’t say I really care about this)
More emphasis on car collecting
The bad:
Rivals needs a new entry menu to select homologated or nonhomologated for all tracks. Current implementation severely restricts rivals event selection.
Instability in aforementioned background track loading - and occasionally not getting loaded into races, or starting early/late.
Homologation does not adequately balance cars in some divisions.
Leagues allow wanton changing of car such that even in homologated league events, people swap cars constantly to the best car for a given track; defeating the point of homologation imo.
Private multiplayer disconnect rate is truly obnoxious (my club currently has 1.5 hour races and a ~30-40% disconnect rate; out of 20 players, 8 will probably disconnect at some point during the race).
No ability to spectate races.
No ability to manually order the grid in private races.
Poor solutions for corner cutting and reckless driving.
No “blue flag” warning to cars about to be lapped.
Magic tires that work as well in dry as in the wet (no tire selection or need to pit when it starts raining).
No ability to pit for just tires, fuel and/or repairs.
No option to speed up tire wear and fuel loss.
Less than stellar damage model compared to other titles.