Tactical

Do not make a design more complex when implementation of more sophisticated logic can remove the need for hardware. Unless the advanced logic makes the system less robust. In which case, examine the possibility of improved logic with improved robustness. Ex. CEB controller with limit switches vs pressure sensor, or RAMPS with v (P) sensor vs limit switches.
Design in a way that average humans can understand, troubleshoot, and repair.
Make bolts easily accessible.
Create simple geometries, except in 3D printing where complex geometry is not an issue.
Highest authority comes from the 'shop floor' - from people building and using the technology, and ideally those people are the same people who do the design. This is design-build-use integration.

Principles

Break the Iron Triangle with Extreme Manufacturing and Extreme Enterprise.
This is design-build-use integration which is a prerequisite for a democractic society. Design-build-use integration is the most accountable way to develop appropriate technology.

Suggest more OSE Design Principles at the forums.

OSE Design principles are based on OSE Specifications, and the main points are summarized here:

Modular design - we treat parts as modules that fit together so that machine parts can be designed and built in parallel. This is called Contract First Design - which refers to identifying the interface between parts prior to beginning the design of the parts.
Lifetime design - instead of planned obsolescence.