What are your *must have* variables for users?

I run a platform site with users and suppliers, and I just started sending out newsletters, but I forgot to add an unsubscribe button or a "no email" variable to these collections. I am going to revise my setup and ask everyone their preferences, but...

that made me curious.

What are the "must have" variables for users, and things that you should be tracking for each one to avoid upsetting your customers or suppliers. Is there a good place to go to find these?