This is the problem - expectation. While everyone wants to see as many new cars and as many new tracks and new features as possible, the fact is that GT5 was never going to live up to the loftiest of expectations, and I fear that some people have been expecting far too much and were never likely to be completely satisfied with GT5.
About my comment before regarding GT5 not being GT4 2010 edition, I stand by that - GT5 is a different game on a different console, and I also fear that some people believe that GT5 should simply be GT4, but in HD and with bells and whistles... it ain't. While GT5 is clearly descended from GT4, it is still a different game in many ways. I reckon that GT5 will be a huge game, but just like any other gaming series, it was never going to be definitive, and we may well have to wait on the next iteration for some things.