Reversibility

How cheaply a change can be undone, treated as a design property in its own right. It is what lets a team move fast without being reckless: high reversibility means being wrong is survivable, so decisions can be made with less information. Most of modern delivery practice — flags, canaries, small batches, backward-compatible migrations — is the pursuit of this one property, and framing it that way makes the practices arguable rather than ceremonial.