Not just internally consistent, but externally consistent, at which point making this a matter of compliance. External consistence isn't subjective. It's as ob

Not just internally consistent, but externally consistent, at which point making this a matter of compliance. External consistence isn't subjective. It's as objective to that system as any other fact, and is a matter of understanding, not feeling or choosing or designing. If you're building an extension to MS Office, make your app comply with MS Office. Same with Adobe Photoshop plugins. And ultimately same with apps/programs for an OS. If the OS does OK-left, do OK-left. The UI that does not exist is the UI that is performing the best. No one "uses" a UI. They just pass it. If you're thinking too hard about it, chances are you'll make your users think about it also. And trying to get noticed will always be at the expense of usability. And finally, metrics such as consistency, obviousness/coherence, and simplicity are also objectively measurable, as are error rates, A/B tests, and step counts. What is subjective is user feedback and even developer feedback. By tallying what people think of an interface, you've already abandoned the goal of remaining unnoticed, and this is how Frankenstein UIs are built.