The game is centered around a fictional music/automotive event, the eponymous Horizon Festival, and all the sweet jumps and crazy drifts you do around the world work toward building your reputation as a radical driver with the festival's attendees. While it's possible to earn points while driving aimlessly around the countryside, the majority of the points you'll earn will likely come from participating in one of the many driving events scattered throughout the world.
Ralph Fulton of Playground Games was very insistent that Forza Horizon is not, in fact, an arcade racing game. It may take place in an open-world version of Colorado, may be free from traditional Forza career modes and may feature a youth-focused licensed soundtrack (read: dubstep), but the actual driving is built on Forza's physics engine.
The problem with arcade-style racing games, according to Fulton, is that the appeal of racing different vehicles is lost due to the fact that all cars have to handle roughly the same in order to preserve an "arcade" feel. By using Forza's existing physics engine, Playground Games hopes to circumnavigate this problem and preserve the series' pedigree for for automotive accuracy.
That being said, the only car featured in the demonstration I saw was the 2013 SRT Viper, and as it was a hands-off demonstration, I was unable to compare its handling with my experience driving similar cars in previous Forza games. What I can say, is that the Viper did more than its fair share of drifting around Colorado's mountain roads, and that it racked up wicked popularity points doing so. See, in Forza Horizon, you're not just driving around for the hell of it. You're driving to get famous.