The course makes is a huge reason that I purchased the game on release day. Realizing it was not there was essentially heartbreaking. Yes, I had a new game, but aside from the physics updates it was basically GT5 again.
As mentioned, the course creator, even as implemented in GT5, adds longevity to the game. The notion of a GPS based creator really peaked my interests, as I'd love to map some local roads that I never got around to creating as tracks in rFactor or GTR2.
The ability for players to take their favorite driving road back home, digitize it and then tear down it in a 458 Italia without the restrictions of real-world driving laws is an alluring one for sure. Being able to share that content with your friends and players across the world just makes it an amazing feature that the community would surely embrace.