The tuning system. GT2-4 had for me, the best car tuning system. I heard that with Forza 2 and 3 it was pretty good, but I've been taking kind of a life break from racing sim on console games. Anyways, on the strength of Top Gear branding and car collecting, I've bought Forza 4. I already love it for one simple reason:
After buying all the upgrades and tuning my car it re-calculates basic figures like G force for me. That is amazing. I used to use small ammount of common sense + testing a car out on a track over and over in the GT games. It makes me smile knowing I can update some figures then have the game auto recalculate telling handling stats about my car.
Also my first gift car was my real life car, that's sweet.