Interaction design laws
Tesler's law
Also called Law of conservation of complexity
Every task has some complexity that can't be removed, only moved: either the product handles it or the person does.
Sending an email needs a recipient; booking a flight needs dates. Someone has to deal with that complexity, and good design takes as much of it as possible away from the user.
Ask who carries each piece of complexity. Engineering effort spent once is usually cheaper than the same effort spent by every user, every time.
In practice
A calendar that works out time zones for a meeting invite, instead of asking the organiser to convert them.
Common mistake
Simplifying the interface by hiding necessary choices, so the complexity comes back later as errors and support tickets.
Where it comes from
Attributed to Larry Tesler, a computer scientist at Xerox PARC and later Apple, who argued in the mid 1980s that complexity is conserved.