Ask someone who doesn't know how to program to read just a few lines of code. Though it's very naturally readable to you -- and in general, if you didn't know a new language you could try reading it line-by-line like English -- it looks totally impenetrable to them. I've heard people tell me that they have no idea how I do anything [programming] when they look at just a simple script in PHP or Ruby.

Knowing the Photoshop interface is clearly not what makes you a good designer, but in general it is a necessary-but-not-sufficient prerequisite to being really good.