The track list in Shift 2 is very impressive, but it's not a simulator, it's a long way from Gran Turismo 5 in many respects.
Speaking as someone who has never played every single GT game, I'd personally love to have all the old tracks in GT5, as well as a few real ones like Pikes Peak.
Before GT5 I used to play Forza 3 a lot, which to be honest has spoilt GT5 for me. If you look at the real track list for Forza 3 and then look at GT5's track list, you'll notice a LOT of tracks are in both. And once you've done 100+ races on Tsukuba in a few months on Forza, then you get to GT5, you're already sick of the tracks before you start.
There's a good quantity of original tracks in GT5, but once you've raced them all since release you find yourself quickly wanting more. This is the reason I stopped playing Midnight Club LA - once you've unlocked everything, you can have every single car, and that's boring once you have them all...nothing to work for.
Sorry for the long post, but I'm bored!