Despite owning all four Gran Turismo games, I am not familiar with the series at all, so I can't comment. I will have to load up GT4 soon and see about the complaints you have about the series. I can't imagine I'd like it as much as Forza though... I admit though, GT5 looks SWEET, I just hope it doesn't suffer from the same thing you mention. Chances are it will though, since they don't like to change a 'good thing'.
Glad you like Forza though. If you want a challenge, work your way up through the Arcade Time Trials mode... it's pretty intense.