Almost everything to be honest. Most obviously, hairpins are always the most boring kind of corner and the fact there are 75 doesn’t make any one of them more interesting. Normal cars tend to trip and fall through them, understeering unpleasantly. Even in a properly set up sports car, endless hairpins can still make you bored almost as quickly as they can make your passenger sick. Yes, if you’re in something like a Caterham with a tight diff and a lot of power, you can have fun swinging the back around but even this novelty can soon wear off.
Actually, that’s unlikely to happen because even before you tire of the child’s play that is sliding a car at 20mph, something’s going to get in the way. If you’re lucky, it’ll be another car, if not it’ll be a peloton of cyclists. Bike riders love the Stelvio because it’s the highest pass in that part of the world and they all flock to its base to see if they can conquer it. So they might be all doing 3mph at the exit of the corner through which you’re now deftly applying armfuls of opposite lock or, far more frighteningly, they’ve reached the top already, and are now descending in free-flowing, loose formation at 40mph.