It's Forza but it's not
The Forza Mortorsport series is best known as a simulation style racer, and it has been since it's first appearance in '05 on the original xbox and it had 4 very good iterations sticking to the simulation formula.
Forza Horizon however should not been seen as a sequel to the franchise, it is in many ways it feels like it's own game not derived from Forza, but a collection of the best parts of other racing games. Forza Horizon takes the festival and atmosphere from Dirt 2 which helps remove it as a serious sim racer and provide a more lighter feel to match the game play, also taken from Dirt 2 is the handling model which is more arcady than Forza's but still requires alot of finesse and where the brakes are an essential parts of getting round a circuit and winning and not simply the button to start a drift. The open world of Forza Horizon has a similar idea to Burnout Paradise's open world with events dotted around the map and more events opening up as your progress, events that range from standard lap races, point to point street races that happen on open roads with traffic to worry about, showcase events which are grand Top Gear like events where it might be a race between a Ferrari F40 against hot Air balloons.
All these events however work because of one thing that really stands out in Forza Horizon, and that is the world. The roads of Forza Horizon are the best i've ever seen, ranging from fast sweeping coastal roads, technical drifting mountain roads, long straight wide highways, bumpy dirt roads, small grid iron towns and so much more. Each road is pleasure to drive whether in a VW Rabbit or a Nissan GTR.
This however brings me to maybe the only downside i've found in Forza which is the car selection. While the selection is still extensive (i believe i heard 80 being thrown around) it somehow feels very limited partly because some manufacturers get complete sets like ferrari and lamborghini while companies like Nissan and Toyota get very little, some odd choices like the Eagle replica of the Jag E-type is present while cars like the Ariel Atom are completely absent.
Overal Forza Horizon isn't really a Forza game, but it's completely deserving of the name that has come to represent a great series of racing games, this is game carries on that tradition. Excellent world design, solid soundtrack, great feel and handling, I can't recommend this game enough for those who just want to kick off their shoes and race.