"Description is what makes the reader a sensory participant in the story. Good description is a learned skill, one of the prime reasons why you cannot succeed unless you read a lot and write a lot. It's not just a question of how-to, you see; it's also a question of how much to. Reading will help you answer how much, and only reams of writing will help you with the how. You can learn only by doing."
– Stephen King, On Writing
All of us, at one time or another, have probably sat through the same English class. You know: the one where the teacher got you to write a paragraph describing an everyday object in new and unusual ways. The one where the difference between similes and metaphors was hammered out as laboriously as your homework on a Sunday afternoon.* The one where you had to come up with better synonyms for overused adverbs like 'quickly' and 'loudly'. (No-one told you that in ten years' time you would come to look upon all adverbs as a thing of evil.) At the end of the lesson, you left with a head full of purple prose and the vague feeling that you'd never be able to look at a cloud again without seeing sky-sheep grazing on an endless field of cornflowers, but you'd also absorbed one key message.
Description is Important.
What you probably didn't learn at school – or at least, I certainly didn't – was when and where to use these new-found skills in a piece of fiction. Personally, back then I was under the impression that the right thing to do was start a chapter with a nice long description to set the scene, before launching into the action. And indeed, I can remember reading plenty of books at the time that followed that kind of structure. If the description was too long then I'd skim it (being more interested in plot than beautiful words in those days), but nevertheless I thought it was the convention. Write the opening description, write the following action, then stitch the two together.
Whether that was true or not (and I think it becomes more so as you look further and further back in time), things have now changed. The move away from third-person omniscient and towards third-person limited – the tendency to stick to one character's viewpoint for the course of at least an entire scene, rather than hopping from person to person – means that this kind of description is usually no longer appropriate. The omniscient narrators of the past would describe a scene from their own perspective and with authorial asides to the reader; they had no problem intruding upon their own narrative. Today's writers, with close POV as their mantra, don't have that option. It feels artificial for a character to stand around describing the landscape for several paragraphs before getting on with whatever they're doing. Description has to be integrated into the character's other experiences and perceptions – and it has to be specific to that character.
Of course, this doesn't mean being able to write good descriptions is no longer important. In fact, it means the exact opposite. Authors can no longer indulge themselves in two pages of clever wordsmithery about exactly how dark and stormy the night was. Rather than being separated from the characters, description becomes part of them. In a close third-person or first-person novel, good description is both brief and idiosyncratic. It gives the reader a mental picture of what's being described, but it also reveals something about the character whose viewpoint it's written from. And that's actually a lot harder than the kind of description we learned at school.
These days, we can no longer treat description as being distinct from a character's thoughts, dialogue or actions. When writing close POV, all these things stem from the character herself. Anything that's described is described for a reason: because it's what was important or noticeable to that character at the time. Someone who's in a strange place will observe all kinds of details, particularly those that differ from his previous experience. Someone who's afraid for her life will focus on what might have an impact on her chances of survival. Someone who's deeply upset may very well be more caught up in himself to the exclusion of the world around him. And so on. As for comparisons, take this: there was a metallic scraping sound, like a sword being drawn from its sheath. All well and good, you may say … unless the POV is that of the archetypal farmboy who's never seen (or heard) a sword in his life. In that case, even if you know it's a sword, he'd be much more likely to compare it to a chinking bridle or a piece of machinery.
In short, to all questions of description – what makes a good description? how frequently should I add them to my writing? how do I know if my descriptions are too florid or too sparse? – there is now a single answer: it depends whose viewpoint you're writing from. Know your characters inside out, and everything else will follow.
* See what I did there?
Write Every Day: tip of the week
Take a short descriptive scene that's written from one character's point of view, and try writing it from the POV of another participant in that scene. How do the descriptions vary? Do the two people notice different things, or the same thing in different ways? Are the descriptions appropriate to their personalities and previous experiences?