I also find that some tracks are simply horrible to drive on. Especially when they use ISI trademark sine wave bumps. I modified my terrain.tdf so these crappy unrealistic bumps are turned off. On tracks that have their own tdf, I tend to turn hem off as well.
Also, some tracks have ''grip zones'', often overdone. Then there are many tracks that simply don't have proper transitions in road camber..
The difference is staggering between a bad and a good track, not just in the 'nature' of the track. At the Ring for example you bounce all over the place over the sine wave bumps and big polygons. At Barcelona you have aaaaaaaaaaages to slowly smoothly drive it..
At the moment I love Road Atlanta by Uzzi. Snetterton 06 is alright, though some parts have a deliberatly 'gripless' outside of corners.
Good tracks in rFactor are few and far between.