The thing I found impressive there is the sound. In Forza 4 it sounds pretty much the same in cockpit as it does on the bumper. In forza 5 the cockpit sound sounds like the sound is coming from outside the car, and then it is ramped up in bumper view. It may seem like a small thing, but that small thing makes the cockpit more atmospheric.
Visually the difference is glaringly obvious. Wonder what forza 6 will look like ? going by the vast improvements last last gen from F2 to F3, it should be pretty big.
I'm almost sure we'll get weather, and it'll definitely look better. I honestly think every single forza has looked drastically better than the previous, whether it be generational or in the same gen. Forza 3 to F4 was a huge jump to me, probably about even for F2-F3. F2 and F5 looked way better because of the hardware upgrade obviously.