Is there any good theory about it? What elements should one consider?
User Interface / Cognition / Latency / Anything else?
Is it only on the side of Non-Functional Requirements? or it lives also in the Functional Requirements side?
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I find that articulating the goals of the UX upfront, agreeing on measurements and goals helps a lot. What is the business case of the UX? How does the business track success? What parts of the experience most critically contribute to this success? What those are really is a case by case basis.
I guess customer UX expectations are much more about politics and rhetorics than about UX itself.
Personally, I prefer a data-driven approach where one backs up the claims either by statistics or (perhaps informal but representative) user tests.
I do like models and I do like cognitive models, but as arguments, it seems people - esp. customers - tend to handle them roughly equal to opinions and rule of thumbs, and they're often applied in wrong contexts.