If your UX asks the user to make choices, for example, even if those choices are both clear and useful, the act of deciding is a cognitive drain. And not just while they’re deciding… even after we choose, an unconscious cognitive background thread is slowly consuming/leaking resources, “Was that the right choice?”
If your app is confusing and your tech support / FAQ isn’t helpful, you’re drawing down my scarce, precious, cognitive resources. If your app behaves counter-intuitively – even just once – I’ll leak cog resources every time I use it, forever, wondering, “wait, did that do what I expected?”. Or let’s say your app is super easy to use, but designed and tuned for persuasive brain hacks (“nudges”, gamification, behavioral tricks, etc.) to keep me “engaged” for your benefit, not mine (lookin’ at you, Zynga)… you’ve still drained my cognitive resources.
Kathy Sierra: Your app makes me fat