Not so long ago I came across a thread, where a Linux newbie was trying to engage the experts by asking "What are the most useful commands at the terminal?". The post language was slightly off, as you would expect from a non pro, but the understanding was functional if wrong. It didn't take long for a - polite and helpful - poster, to point out the errors in understanding, and this was when I couldn't help myself but think: With friends like this, who needs enemies.
Don't get me wrong - I like Linux and how the basic theory of operation is simple and consistent, which makes for few surprises and obvious fixes when you run into surprises, but insisting that any user needs to grok the theory of operation is the same as saying no to UI abstraction. Which in turn is the same as saying no to non-techie users. Hundreds of analogies from knowing how the electrical engine in a blender works to knowing how to repair your car engine comes to mind. None of this helps you blend or drive. And yet, it clearly helps you run Ubuntu to understand Unix basics.
The "simple theory of operation" combined with "minimal facade" - two goals that makes it easier to be a super user, are at odds with the experiential goals of any non techie user. Which is why the only way non-techie users get to enjoy these platforms is through the loving help of techie relatives (so the users Linux can get are the ones who just need an info appliance that has basically been done for the last 10 years. This appliance is so usefully shaped, through wear, that it can - even on Linux - be used as is.
Linux has a UI chasm: For users needing only 1995 computing, it's fine. For super users it's fine. For savvy inbetweeners it's just not there.