The changes made from Japan's version of Sailor Moon to the one shown in America/the West.
The travesty that is Black Butler and 07 Ghost, especially the latter since the manga is relatively short. Dunno why they didn't just...follow the manga instead of making some fluffy fan-service-y crap for the majority of the anime, smh.
But on the most basic level, I'd definitely say the voice acting skills and the tendency to censor things to fit the culture they're being dubbed into, even if it means losing something that's important to the original.