What is the One Most Important Parallel-Programming Lesson?

Given a proficient sequential programmer, the single most important lesson on parallelism is shown below.

On hot code paths, avoid global locks and updates to global variables:
Do not lock code

Instead, lock data rather than code, and partition oft-written data across data-structure instances, across CPUs, or across threads:
Instead, lock data

Is this all you need to know?

Absolutely not!!! But it an excellent place to start.