This is half meant as a reply to Nerf NOW!!'s "Skill Issue" comic and half meant as an idle thought. For context on the former, here is the comic's associated blog post:

Artificial difficulty is the name we give for difficulty we don't like as opposed of natural, organic, shade growth, GMO-free difficulty.

I think "artificial difficulty" is a real thing, but the lines on it can be fuzzy. I think the easiest way to define it is that "artificial difficulty" is difficulty that can't be overcome through improving your skill, or difficulty that breaks the rules established by the game. I think it correlates with lazy or poorly thought-out mechanics, such as bullet-sponge enemies and trial-and-error puzzles.

I say that it's a fuzzy line because the same mechanic can go either way. Take the ancient dragon fight in DS2: It has the highest HP in the game and can do a massive AoE attack. Would it be made harder if its HP was doubled? Sure, and you could argue that it requires more skill to fight him because you need to know how to stay alive for twice as long. But what happens as you triple his health? Multiply it by 10? By 50? By 100? Eventually it goes from a test of your reflexes to a test of your patience.