First: Fast, Isolated, Repeatable, Self-validating, Timely.

Even a properly structured test (AAA or GWT) can be useless: it is slow, environment-dependent, or requires manual verification. FIRST helps to avoid this by setting clear quality criteria.

The tests must be as fast as possible. They're supposed to run by all members on the team for each task, there can be hundreds of them for each commit.

The tests should not depend on one another. Any dependencies to the database, network, clock must be emulated by the interface.

A test must give the same result in every environment.

Tests must be able to auto-detect if it passed or not.

The tests must be written before or during production. Otherwise you might end up testing only positives, forgoing testing the actual errors that influence production.