I think GT5 really fucked up. They took 5 years to develop a game that is obviously great but not revolutionary in any way. Polyphony should have spent less time nitpicking minor details/pointless additions and gotten this game out way sooner.
They allowed 2 Forza games (both very high quality) to come out during the time of its development and then when GT5 finally comes out its getting lower scores than Forza 3? Come on Polyphony, you gotta know you messed up.
I think GT5 could have easily come out in 2008 with 1000 cars (none of them "premium"), all the tracks/races it has now, the great graphics it has now, online, plus better damage modelling...but without un-necessary stuff like a track editor, go karts, premium cars, b-spec, etc. It could have come out in 2008 which would mean much lower expectation and it probably would have pulled 90's across the board by reviewers.
...Then saying GT5 came out in 2008, they could start on GT6 which would add all the un-necessary features GT5 has but to an even higher quality and it could come out in 2010 or 2011. That way there would be a real GT5 game out (prologue doesn't count) to counter Forza 3 and then an even better GT6 to kill Forza 4 whenever it comes out.
That's just my opinion. All that being said I'm gonna go buy GT5 probably this weekend.