Purple prose is generally tedious to read because it's trite, not because it's too descriptive.
Take a writer like Nabokov: guilty of florid, ornate, self-referential, beautiful prose. He makes it work. He was the rare type of writer who could wax rhapsodic about the mundane and make it transcendent.
If prose is rife with cliche, inept or trite metaphor, abuse of adverbs, and various other sins of overwriting, then readers will accuse the writer of purple prose. But the core problem is probably triteness.
Definately this.
And even when you look at authors who are particularly lyrical, they still have clean, clear language. Adverbs and adjectives should only be used if they add something unique, or a particular rhythm.
Also, be concrete, rather than abstract. That is at the heart of good "showing".
Adjectives and adverbs works best when they are used with absolute precision yet are a little suprising. Ask yourself "have I heard this description or turn of phrase somewhere else before, or is this new?"
Same withe metaphors and similies. A few very unique ones go a long way. Don't feel like you have to struggle to come up with a lot of them.
Avoiding purple prose doesn't mean dumbing down your language; as a reader I love beautiful, complex prose, and I am not averse to the occasional trip to the dictionary. But those rare words have to be used very precisely, cleverly, and be absolutely necessary.
For example, I remember in an early chapter of
Kavalier and Clay, Michael Chabon refers to "all of cathected Prague." That is a brilliant use of a somewhat obscure Freudian term. No other simpler word would do precisely what "cathected' does in that sentence. It also fits because the character his omniscient narrator is talking about is a psychologist, back in the day and age when Freud was a big deal, and it fits the overall themes of the book, about how people deal with their loss and unbearable attatchments through escapism. So the rare word works because it connects with so much in the book, rather than just being an instance of going to the thesaurus for something new.
I would read some authors who write great prose to get a sense of what good lyrical writing sounds like. Don't try to copy, but examine how they use metaphor, simile, and other aspects of language to paint beautiful pictures, and how it differs from trite, purple prose. Some of my favorite books to study for the prose and they way it fits the narrative and the characters are:
- To the Lighthouse by Virgina Woolf (more complex...but not necessarily with big words),
- The Road by Cormac McCarthy (lyrical but sparse),
- The
Earthsea Books by Ursula LeGuin (clean but also lyrical, an especially good example for fantasy prose),
- Little, Big by John Crowley (more ornate, and requires an occasional trip to the dictionary, but a great example of how to do ornate prose and big words well.)
It does also depend on POV and what type of book you are writing. If you are writing from a character POV, your language should more closely match their thought. If you are writing from an omniscient POV you have more freedom, writing in the voice of the narrator, but still want to keep it appropriate to the style of the book and the characters (and your narrator's attitude towards them). You want your prose to "fit" your story, and your genre.
If you write fantasy, Ursula LeGuin's essay "From Elfland to Poughkeepsie" is a must read about prose specifically in fantasy. It was written a few decades ago but is timeless advice.