Well 10km max track length is a real downer. With all the talk of 10X10 km area to create a circuit in, I was expecting to be able to create far larger circuits than 10km max length.
Doesn't affect me anyway as I won't be playing GT6, but I hope they make the max length much longer for GT7. I remember the first track on my list, back when we all thought the course maker would only take a couple of months to come out, was the original Spa-Francorchamps. But, at 15km, it's 50% larger than the max you can make.