Ever since I was a kid, writing has been my dominant mode of expression. I've always been enchanted by the written word and the complexities of accurate description, the nuances of voice, and how your audience affects what you write. Obviously, you don't have to have a degree in writing to teach an online course, but when you develop one, you've got to write a lot - from clear assignment directions to explication on content, in an easy to follow way, to communicating through emails, threaded discussions and chats. It's important that you have some flair for writing. The most successful teachers I have met are those who like to write.
Good writers are sensitive to their audience.
To create an extraordinary online course, you must be sensitive to your students' perspective. You've got to be aware that they can't raise their hands and ask you to clarify something immediately and how frustrating it can be when, as a student, you're missing key information. Or, as I have seen, a mistyped page reference to a reading assignment can send students into a frustrated tizzy. Another aspect of being a good writer, and thus a good online teacher, is the ability to organize. Designing and teaching an online course works best when you can outline and develop the bulk of the course before it is actually scheduled to run.