Going from a GNU system to a non-GNU system

What do I need to know? What habits do I have to break?

Non-GNU systems tend to be more consistent, more reliable, and generally better-designed.
You will be expected to read the manual, and non-GNU man pages are actually useful, rather than just annoying advertisements for that GNU info shit. Expect to be publicly ridiculed if you ask questions that are already answered in the manual.