I've read the list a couple times now and that #11 is the one that just isn't that substantitive to me. I understand what it is saying intrinsically, but I'm having a hard time wrapping my head around it firmly.
Lemme take a stab at this.
Your job, as the writer, is to give to the reader important information about your story in the most efficient (and hopefully entertaining) way possible. For example, in my story I have a kitchen with yellow dining chairs. The chairs serve no real purpose in the story and the color of the chairs isn't important outside of scene setting, and my MC (main character) has no special relation to the chairs. In short, they are nothing more than scenery.
The most efficient way for me to write about the chairs would be: The dining room chairs are yellow.
Simple, clear and to the point. I'm sure, right now, you have an image in your head of a yellow chair. Mission accomplished. Are these highback chairs, cushioned chairs, or plain chairs? In my mind, the detail is plain, but in your mind, the chairs might be something else. That doesn't matter, the only specific detail I wish to impart is the chairs are yellow.
Now, let's take the same situation and break Guthrie's rule #11:
The dining room chairs were the color of molten gold, faintly reflecting the light in their simple, clean elegance. Sometimes the light would shift and the chairs would take on the hue of amber. Especially at sunset, the chairs seemed to grow older and more dignified, as if growing wiser with the setting of the sun.
See, here I've just spouted off a lot of random words that serve no purpose to the story. Its unnecessary exposition and kills the writing. Its an attempt to "sound writerly" by overcompensating.
Around a year ago, there was a member that remarked novels were nothing but 90% fluff. It is this attitude that leads to overwriting: the idea that everything in a novel or story must have some deep description or detail.
Uncle Jim gave the Mark Twain quote (I think Twain actually said "Eschew surplusage", but its essentially the same thing): "Eschew obfuscation". Why say "Eschew obfuscation" when "Be clear" works just as well, reads better, and is most likely more easily understood by the masses?
TLDR version: Don't try to show off your vocabulary or mad writerz skillz. Just write so people understand what you want them to understand.