Wheels, standards and premiums......
First up wheels, In Forza wheel changes effected weight, thus performance, in GT they didn't and don't in GT5, it's just for that custom look. Dose it effect the game, no, if there were no wheel changes available at all would it even get mentioned, no. Can you see your own wheels when driving? no, You can see them in photo mode but thats an added feature (a good one) and one that has no direct bearing on the core of the game which is driving.
Premium V's Standard......the big thing here is the cockpit view.....personaly bumper cam gives the best view to race in, you can see the apex more clearly and can place the car more acuratly, but thats a personal thing. In previous GT games there was no cockpit view, and in Forza 2 there was no cockpit view whatso ever.....At the time other car sims had cockpit views, but it didn't stop Forza 1 &2 being stunning games nor did it stop GT1, 2, 3 and 4 also being stunning games without it!
I must point out back on the PS1 the first NFS had cockpit views! and that was out before GT1.....cockpit views are not new but nor are they the most common veiw point to race in, there nice but not a necesity for most.
Graphics........Ok standard cars don't look as good as premium cars......yes thats a fact, they don't look that far off and in race conditions there's no imediate difference. But standard cars are still better looking than any other car in any other console racing game!
Quick point at the end. the best "sim" on the PS3 would be one of the ferrari challenge titles........people love it, I love it, but if you look at the cars on offer, the tracks on offer and the (sorry) pitiful looking car models you would be hard pressed to see why people like it so much.....until you actualy drive the cars in it........that is the same with GT5, if you look at the physics of GT5P and the GT TT there physics are even better (GT5P debatable but GT TT definatly) and thats the whole point, it's a driving game, forget the whistles and bells, get in there, drive it and remember why the GT series is renound for being the title that allways pushed the genere forward every time a new incarnation was released.
I'll probably have a Maserati, Tesla or Challenger from GT5 eventually....maybe an ACR Viper Red Challenger with a carbon fiber hood. 
