If you listen to exponents of the "plain English" school, you would infer there are two ultimate factors that determine the level of difficulty of a piece of writing: inherent difficulty of the subject matter and amount of artificial complexity imposed by bad writing. Hence, the common advice that writing should be as difficult as the subject matter requires but no more.
Complexity in writing is more complicated. A writer tailors his writing for his audience by modifying the three ultimate factors' weights. While submitting to Concision's pre-eminence, legal writing places great weight on Clarity. Concision and Clarity eventually conflict, and the writer will settle such disputes more often in favor of Clarity when writing a legal brief, more often for Concision in blog writing.
Overselling Clarity ignores the reader's motivation, whether the motivation to read as many words as the prolix writer adds or any part of a dull-sounding, uneuphonious writing. The weights that the writer should place on Clarity or against Concision and Euphony depend on the intended readers' motivation. An audience's motivation remains important despite its captivity, since reading can be more or less thorough, more or less sympathetic, and part of this adjustment occurs outside the reader's awareness. Still, motivation becomes less important, and with it, the writing Virtues that drive reader motivation, Concision and Euphony.
Readers call writing difficult to read when they prefer more Clarity. But writing becomes less clear not only when the subject matter is difficult or flaws conceal meaning. Writing also becomes less clear because Clarity compromises with Euphony and Concision.