From what I've read from the reviews a lot of Forza 3 cons could be applied to the GT series also. That's is you get tons of cars and tracks yet the races themselves are hard to balance especially endurance races. Most often with most race games either I have won or loss the race in the first few laps.(more laps only delays the obvious win or loss) With FM or GT either the races are easy to win with a variety of cars especially with upgrades or so hard that only one car fully upgraded can give you a chance to win. (If only a few cars are actually useful in winning races then that kind of kills the point of having 400-900+ cars .)
While GT3 had a over all higher score than a game like Nascar 03 I actually enjoy and spend more time playing Nascar 03 than GT3. Nascar doesn't have tons of car but I had a lot better door to door racing especially with it's career mode. (Strangely EA has done a half-way job on the more recent versions of Nascar.)
Now that games like GT and Forza has been done so often you will more likely to find reviewers not as excited over Forza 3 or GT5 like they would when GT3 first came out.
