You can figure out what you ought to do when writing a look at what writers do. It’s harder to figure out what you shouldn’t do, if the writers you read don’t do it. This is a “shouldn’t do”.

I wrote this yesterday for the intro to “All the Pretty Pony Princesses” (or, “the season 3 finale as written by Joss Whedon”). This morning I realized there was something wrong with it.

The mayor still hadn’t looked at her. She kept frowning at Rarity, who had the mayor by her left forehoof and was leaning in close to tell her something, then at Fluttershy and Applejack, clustered together on her right, and then down at the hard chalky dirt at her hooves. She didn’t look at Rainbow or Pinkie at all. The two police mares with her studied Twilight guardedly. They were probably intimidated by the bright Element of Magic around her neck, flashing in the noon sun, which they kept glancing at.

This description brought me up short when I reread it the next day. It seemed to jerk to a stop when it mentioned the Element of Magic.

I think this is because describing a scene from third person limited point of view is following one character’s thought process. The description can end before getting around to everything in the scene, but it may end only at points where a person’s thought process would switch to something else. No one would turn their attention to what the mayor is looking at, down at the ground, then to the two police and what they are looking at, and then stop on reaching the Element of Magic without that leading to any further thoughts, at least not unless they were interrupted by something urgent.

I added this at the end of the paragraph:

They were probably intimidated by the bright Element of Magic around her neck, flashing in the noon sun, which they kept glancing at. Twilight had hung it on a string that Rarity had thrown in the trash, just until she could find a proper gold chain for it.

Note that I didn’t describe the Element! But I had to go somewhere from there; I couldn’t end the paragraph on something that no one would end a thought on. Long descriptions should either break when they are interrupted by a story event, or else trickle off when they reach a point where a person's attention might turn to something else.