I have played the game for more than 50 hours. No question the graphics and driving experience are better than Forza 3. Some problems that carry over:
1. Many of the cars have no rear view mirror visible at all - the outside mirrors are beyond the limits of the 16:9 1080p screen I have.
2. Lap times frequently do not update on the leaderboards, nor on the in-race timer. So, you do a lap of 1:53.042 as your "last lap" and it shows your "best lap" is still 1:56.317, for example.
Some new irritations:
1. The races are much much shorter on average. Even for the S-class and above classes they are a maximum of 4 or 5 laps vs. 8 to 10 in Forza 3.
2. While there is this career "global" mode, there seems to be no rhyme nor reason to it. There is no sense of racing for a season championship the way there was with Forza 3 as you experienced 15 R1 class races during a season interspersed with other, lesser classes. It would be nice if they did a true simulation of a LMP season, the way F12011 does. As it stands, this is just a series of short races in a variety of cars, not a true racing sim.
3. No Porches - not a big deal for me, but may be for some.