here is an idea (hopes yamauchi reads this)

lets say u come back from work, turn your ps3 on,slot in your gt5. its nite time. all of your racings, the time is nite. so if you want to race daytime, you need to play during the day(ponteng work or weekends)

now thats a feature. or lets say, the weather forecast in nurburgring is partly cloudy. so if you choose to race there, you will see cloudy weather. real time weather effects ya? omg, what if it rains...or snowing? now thats hell man.