Registers being different levels of formality (informal, formal, slang, etc)? If so, Cantonese has that in spades.
Your answer also ignores the tonal nature of Cantonese/Mandarin. Most guides can't event agree to how many tones there are. I get by with 7, but it can range from 6 to 10 depending on who you speak to (in contrast to 4 in Mandarin).
I thought of mentioning tones, but they're not as much of a problem for learners as they're made out to be. Cantonese has seven tones. However, there are no word pairs where the only difference is that one has a high level tone where the other has a high falling tone.
On a lesser scale, the hardest thing about learning German, as, again, an English speaker, but one who had poor grammar instruction and no grounding in Latin, the idea of cases for verbs and pronouns was... weird.
I've had a bunch of German, and just passing contact with Latin, but a lot of German grammar started making a lot more sense when I started thinking of those languages as similar in a sense, because of declension.
True that. Also, it's just a different use of tone - English uses tone to differentiate questions from statements, and to otherwise add meaning to words/sentences. There are other mechanisms for that in Chinese dialects.
"Really?": Convince me; "Really!": I'm indignant on your behalf; "Really hungry": exceptionally [hungry]; "Really good": average.