I hope to see many replies here from many people because as Eyebo said, It's a question with many different answers. Everyone has different tastes. There isn't really a right or wrong answer here. Just opinions.
I agree also with Eyebo's statement that it should feel like the car want's to go there and I will change a corner or section many times to get it to feel that way.
Sython1 wrote:What ballance is there between a hard level and a fun one?
I think a good track needs to be driven a few times to be learned but it shouldn't take so long to master that you smash your keyboard or controller in frustration. I mean after all it's suppose to be fun yes? Putting an obstacle in the driving line to force players to alter their flow can be good if it's not in a "blind" area with no warning. For example I've never been fond of a fast upslope with a hidden sharp turn at the top unless it can be used as a calculated risky shortcut to jump to another part of the track.
One thing I think is important with any track is to test run from every checkpoint. Nothing frustrates me more than resetting to a checkpoint that you cannot complete the track from. IE: a CP at the edge of a cliff or gap you have to jump over without an alternate route.
As I said I hope to see many opinions posted here. I'm curious to see what people like too. I know my trackbuilding style is a little "Old School" but then so am I and what the average driver likes might be quite different. I tend to like two or three lap tracks that run around 4 to 5 minutes with fast flowing sections and tighter technical sections and some sort of unique theme to them. Seems most players like the shorter 20 to 30 second runs though with a good challenge in it.