You cannot over-emphasize avoiding component state

§ 5 does not sufficiently focus on avoiding stateful components.

You should not just prefer stateless functional components over stateful components (classes) whereever you can, but if you need to use stateful components for their lifecycle methods you can still delegate state handling to Redux/MobX or whatever instead of internal component state.

(There were some tweets reminding me of this issue: https://twitter.com/dan_abramov/status/798858012039397378)