Let’s not forget the purpose of designing an interface. Interfaces should be designed to communicate the message of the content, within it’s context, most effectively.
They are not your art-board canvases. Don’t design just for the sake of “prettying up” – that usually leads to over-designing and distilling focus away from the crux of the interface.
From this need to reduce distraction from the content itself comes the phrase “the best design is invisible” – leading to the trending rediscovery of “flat” and minimal design.
Yet people (and by people I mean large corporations included), tend to go a tad too far with this notion of minimalism.
“No UI” implies that there are hidden interface controls that are triggered through user gestures that are not explicitly obvious by looking at the interface.
These hidden controls definitely have their place in the interface design realm. Some good implementations include the 4 finger swipe on iOS to switch apps and the swipe from the edge of the screen on Android’s Chrome to switch tabs.
However, when these two concepts get convoluted and misunderstood, you get beautiful tragedies like Windows 8 where the main interface controls are hidden from sight.
Hidden controls and gestures are handy as quick extensions to visible controls - which are usually only useful once the user is comfortable with the interface. If a distraction-free environment is a crucial part of the experience, it should be handled with Progressive Reduction of the interface - not vanished altogether.
Timo Arnall goes into much more detail in his “No to NoUI” post to give a much better understanding of this trend.