Actually, there are plenty of keys to good writing, but I'm going to focus on one of the most basic--Reading. Yup, that's what I said. Reading. If you don't read often and in a variety of subjects, your writing will suffer. The following are some great reasons why reading is critical for writers.
Reading is a necessity to teach good writing mechanics (as a supplement to what you learned in school, of course ;). The more you read, the more you'll absorb proper grammar, structure, and punctuation. These are the critical tools to building a solid foundation for your writing project, and a good working knowledge of them is important to produce smooth, flowing literature in any genre. Not every book you read will be a sterling example of good mechanics, but this is also a great learning experience.
Another reason highlighting the importance of reading is the creation of style. Every writer has their own literary voice, their own word flow and method of storytelling. Most writers aren't born with a style, though. It must be cultivated and tends to evolve over time. Reading copious amounts exposes writers to a variety of styles, to see what works and what doesn't. For aspiring writers, this provides a great base to create their own style. For veteran writers, it's a way to confirm their style, to keep it fresh, three dimensional, and engaging.
Reading also provides inspiration and a great working knowledge of a writer's chosen genre. If you choose to write fantasy, but don't know much about the lore of typical fantasy creatures, your writing will show it. The creativity and inventiveness of fiction writers is endlessly amazing and useful, giving loads of story and character ideas. Of course, I'm not advocating plagiarism, but concepts are everyone's domain, and even the smallest detail can blossom into a full blown story idea.
For writers who are also concerned with the business side of things, reading literature that is currently popular can give them an idea of what their target audience is looking for in a book. This can influence a writer's choice of genre, subject matter, and even character type and personality. I'm not saying writers should shape their entire writing project based on the books that make the best selling lists, but it can give writers an understanding of their audience.
If you're a writer, make sure you take the time to absorb some literature. The world is a big place, but reading opens up a universe of creativity and provides the backbone to good writing.
2 years ago