What should an American know before moving to Hong Kong?

It's long been my dream, after graduate school, to live in Hong Kong for a year or two. What can I expect? How is immigration? What is the situation in the finance or business sector? What should I prepare for and bring with me? What American things should I NOT do there?

Thanks Redditors!

EDIT: I've got some conversational Mandarin. I'm still learning and improving. How much will Mandarin help me in the Cantonese-speaking city?