What makes a great driving game?

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Hi guys! Just doing a little research for a project and wondering what you all think it is that makes a great driving/racing game?

Is it the feel of the cars, the biggest library of vehicles, accurately-rendered dust particle effects, maybe a great competitive multiplayer? What is it that makes GT and Forza that much more respected than SHIFT?

I was also wondering what your favourite driving games are? It doesn't matter what platform it was on, I'm just interested to see what games people love! =]

I have to say that my favourite driving game was probably Gran Turismo 2. The sheer expanse of the game was mind-blowing and it just felt so good! Though the game was definitely improved upon with the later versions, the 2nd game has a special place in my heart.

So what's yours?