Going from a GNU system to a non-GNU system

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

Expect more shell tools to do exactly what you tell them to, rather than what you mean. For example, rm may not save you if you try to glob something beginning with a dot.

Pretty much all of the common tools will have fewer options. If one of the GNU tools provides something that's only available as a long option, you can almost guarantee it will be gone. This is especially painful where shell builtins like set or shopt are concerned. Korn et al. have their own idioms for dealing with old Bourne shell annoyances.