Oh, probably everything then depending on what your definition of correct pronunciation is. For example, the English name for Den Haag is The Hague. You guys say that correctly, but it's not really the city's name is it. It's just Anglicized so it's easier for you guys. So if you were to try and pronounce Den Haag I'm sure 99.99% of Americans would fuck it up.
Same goes for Amsterdam, Rotterdam, Utrecht, Eindhoven, Maastricht, basically any one of them would be butchered by non-natives.