Words are a beautiful thing. Through them, one can sum up the whole of human existence, explain the mysteries of the universe, invent fascinating settings that have never been seen by any pair of eyes. It's easy to fall in love with words...and it's easy to get carried away when writing them. When you're writing, be careful not to start over-writing.
What is Over-Writing?
Language is rich and full, and English in particular has a mind-numbing collection of words. Certain words are evocative, provocative, offensive, image-inducing...let's face it, words can do it all. But if I use a word like lugubrious instead of gloomy, hardly anyone will know what I'm talking about. I can just as easily use the word invigorating instead of the much lesser-known salubrious.
Yes, it sounds impressive to use more flowery language, but obviously if you stretch your mind to find really uncommon adjectives you're going to frustrate your readers. Not many readers are going to be willing to keep a companion dictionary nearby just so they can get through a book, particularly one that's a fiction novel (which are, inherently, meant to entertain). The average reading level for adult Americans is 8th grade. So if you start throwing words like mordant and cumbrous around, you're definitely over-writing.
There's another reason you don't want to use it: realism. Over-writing is particularly terrible in dialogue, and you may notice that some writers (myself included) make glaring grammatical errors within dialogue. This is a necessary thing sometimes, because above all you've got to write it the way people talk. Most people are perfectly happy to leave propositions dangling at the end of their sentences, and in order to make your dialogue flow better you might have to write it this way. People don't often say words like turgid and obtuse, so your story might come across as a little silly if you've got characters who are using them. There are some very specific instances where over-done language might be an important aspect of a particular character, and in this case you should definitely flex your literary muscles.
Keep It Simple
Above all, your writing should be readable. I can't be stumbling over gigantic words all the time, backing up and taking a second look to try and figure out what it all means. Always check your writing for readability. Read along with your words, so you can hear how they sound. Even without using a bunch of four-syllable words, it's possible to create stunning scenes and vivid detail using simple, everyday English. It is possible to try too hard when you're writing, and when this happens over-writing is a common result.