I love Green Hell. Why does everyone hate it? It's the hardest race track ever. What did you expect?!
I think most people that are complaining don't like the GP version, but love the old school course.
Me? I usually can find a car I like to drive on any course, and the car makes the difference to me. If I am not enjoying a particular car on a course, it is because I am trying to force a car around it that is not ideal for the course. If you get the right car, it makes a course enjoyable. I don't like city races for the most part (Seattle is my exception, and Hong Kong) but most city courses are more fun with the smaller lighter car where you are not pummeling a course with power, but driving it with finesse. Same for Matterhorn. A 71 Mach 1 or a Superbird will make you hate that course. A tuned Miata, BRZ, S2000, etc, will let you enjoy it more IMO.
Most of what makes the GT series so much fun to me is finding the right car, tuning it right, and enjoying a lap done smoothly and feeling that feedback that says you are working out the car as it was intended (and not using it as a battering ram). Forza is more arcade, race hard and slam into things, and I can enjoy that too. But I feel that the GT series does make me work a bit harder to find the right combinations to make the experience more...personal? Exquisite? I will pick up my controller for Forza and do a race here and there as I have a minute or 15. GT5 and 6 I have a chair, a wheel and pedal set, and I am much more immersed in the experience of tuning a car to find the right balance to control it more smoothly and pass without touching the other cars. Doing races over and over to find the right places in the track to pass without touching and win the race.
As far as the OP, I like the differences in most of the tracks. If I dislike SS5, it is because I miss the much longer SS11. If I am grumpy driving London, it is because I want Paris, Hong Kong and Seattle.
Speaking of, I would gladly pay 20 bucks more a game if they had two disks and we could load in another 10-20 tracks. Not 10 versions of one track, but 20 plus tracks that are not added now and have multiple versions of each if needed. add another 10 bucks or more and add a big realistic rally section. One of the things Forza does so much better is many sections of rally races, done as TT's like real rally racing. GT4 had some decent rally courses, but Forza takes it to a much higher level. I want to see PD get serious about rally stages.