Total simulation racers are not something I've played much at all. I've got nothing against them, but I tend to drive like a maniac and a lot of the detailing and physics are like pearls before swine with a guy like me. Reflecting on my preferences makes the existence of a spinoff title like Forza Horizon apparent.
Playground Games, the title's developer, is careful to say this is not "arcade racing," it is "action racing," every chance they get, I can't speak to whether it really is a hardcore Forza Motorsport fan's cup of tea. It seems to appeal to my colleague Jason Torchinsky at Jalopnik, and I'll defer to his car knowledge. You should read his appraisal, and consider mine below more of a companion to it from a more casually interested perspective.