Honestly? Nothing anymore. I'll try to remember GT for the great game it was, but for that I'll pretend 5 and 6 never existed.
Still hoping that GT7 will be the next best thing and that manages to win me back.
Leaving negatives aside, the things I liked about GT6 ( I say liked because I'll probably won't be playing it again) was the following:
-Menu
-Fast loading
-Graphics (something that GT always has and always will excel at)
-Replays
-Physics
... Unfortunately there is so much these things can do to keep the interest in the game.