[octbash]

Besides vocabulary/characters, in what sense is Mandarin not simple?

[2a0c40]

Tones

[kaesar14]

Which is a massive sense in which it is not simple! For any non-native speakers, (I'm a native English speaker), tones were nearly impossible to master, and I still have a terrible grasp of them I think. It requires a complete rewiring of the way you speak language, sure, but it also requires a complete rewiring of how you hear it too!

[jhanschoo]

Tones are simple (in the sense of being governed by linguistic rules), just foreign.