Some people are quite ridiculous with this topic. They think, "Oh, six years and this is all they could come up with?!" Well, what did they come up with? Amazing premium cars, amazing tracks, amazing graphics, and even better physics; the best on the market for this genre.
Comparing Forza to GT is useless. Forza is on the 360. The technological advancement from the Xbox to the Xbox 360 isn't that significant. Let's look at Final Fantasy XIII, the graphics look breathtaking on the PS3, but on the 360, screen-tearing is an issue and the graphics even had to be downplayed for the 360 to process it correctly.
If it took PD to make GT 5 in six years and it was on the same console, then you all would have a proper reason to complain. But this took them six years to make on the PS3. That's a whole new system. Just look at the size of the PS2 and compare it to the first-gen PS3's. The PS3 is absolutely enormous and the technology far surpasses the 360.
Not only did they have to make ideas, scrap those ideas, see what ideas worked, see what standard cars worked, etc.etc.etc. but they had to do so much more. And with all that time they took, they would be losing more and more profit.
tl;dr - Don't be so butt-hurt by the number of premium cars. Any person with a reasonable sense of mind would realize that they took the correct time needed for this.