I was playing with my wife's phone this weekend, trying to find some information and the subsequent aggravation got me thinking about 3 'C's which should really always be obeyed when designing a UI, all of which tie together pretty nicely.
Consistency: Once your user has performed a task, it is crucial that similar tasks are performed in similar ways. This seems obvious, but I see it ignored all the time. One of my frustrations with the phone was that it always seemed like it was a different button I needed to hit to navigate to/from screens.
Clutter: More accurately, a lack thereof. It can be so easy to add bells and whistles and fancy things to a UI which can overwhelm and make it both confusing and frustrating for the end user to accomplish what they are trying to do with your UI. This leads to me a bigger, broader point, which is...
Clarity: The most important thing with any UI, in my opinion, is clarity. It is so important that the purpose and use of any given screen is clear to the end user. When my wife first got her phone she spent 30 minutes on the bus trying to figure out how to unlock it because it was the opposite of clear and intuitive.
Any time your user (and you need to consider all possible users, not just the ones who are already accustomed to the type of UI you are designing) struggles with the clarity, consistency, and clutter of the UI, it leads to a frustrated user who is less likely to continue to use or recommend your product, and less revenue is always a bad thing. Relating back to the aforementioned wife's phone, the frustrations she/we have had will result in me purchasing a different type of phone in 3 months, instead of buying the same phone she has.