For me:
+ Menus: speed and arrangement of items
+ physics
+ lighting, day and night circles
+ career mode with coffee breaks and licences is fun
+ online mode is much better and will still be improved (filtering of cars, forced pit stop, ...)
+ garage: filtering, sorting, loading times, less clicks, details view
+ great new cars and more to come
+ great new tracks
+ The lunar events are draft, but I love the playfulness of it.
+ Matterhorn is gorgeous with excitingly steep slopes.
+Love the new physics.
+Love the menus but they need to be faster in dealerships. Tuning menus need streamlining.
+Lighting mostly works well. Shadows are weird and darks too dark. Ghost car SUCKS.
+ Goodwood is hard but fun.
+ Career mode progression is good but is let down by AI gifting you races.
The feel of the cars. The way the car won't tolerate you locking the fronts and trying to go in too fast is excellent. Comfort tyres and no driver aids... Great stuff.
MB Sauber
The Sauber has basically been converted to premium. I don't think the others that you've mentioned have
In 1 Word Bathurst is the best thing about GT6 , Now all we need is this car >
And look what happens when you hit the tyres at forest elbow IRL ( track has changed so much over the years )