What are some circuit design rules of thumb for mass production?
Improve s/n ratio. Noise reduction.


Most important - avoid or mitigate ground loops from becoming a microwave antennae
Or any loop from becoming a microwave antennae
Reduce line/track impedance (especially inductance) first or otherwise then mitigate its effects
Appropriate line termination to debounce oscillations at start and end of square waves.
Test for propagation of noise from power supply to circuitry
Ensure sufficient power injection points at vicinity of ICs to avoid signal flaccidity due to flaccid response of power supply input.
Match or exceed sinking requirements from sourcing currents.
The lower the power used, the better


Testability and debugability


All active components on topside, most impedance mitigation components at the bottom, so that it is easy to desolder and resolder those passive components during debugging.
Well-defined functional pattern based logic design. Such that the state of the application board can be easily deciphered from signal combinations.
So that either a monitor chip is embedded onboard giving status on LED, or there are sufficient appropriate test sockets to probe signal patterns.


Flexibility


Use either configurability or programmability means to allow board to be used differently.
Use programmability to reduce component count.


Programmable parts are not that expensive. 
Programmable parts already come with functional pattern specs you can test against, so that you can base your functional patterns on the programmable part. 
Why not acquire expertise in embedded programmable ICs?

Design/embed impedances into the circuitry. (I don't know much about this and don't know how viable it is.)