Know the old school basics of design.

Limits and fits, tolerance stacks, good drawing practice, know which dimensions are important in your design and which are not and tolerance accordingly, etc

Also, know how stuff is made; how mills and lathes cut material, what are your machine capabilities. The number of, unrealistic tolerances and impossible geometries I see from new engineers is astounding.

Also a good understanding of statistical analysis tools for optimising assembly designs.

Sorry if these seem boring. But they the foundation of mechanical design and appear to be seriously lacking in new engineers.

I have just left Reddit and deleted my account, on account of this.

Fuck Reddit.