If you had to list your tasks in order of difficulty, where would writing job descriptions land?

That depends on the personality.

Job descriptions is one of the very essential part of HR and also business documentation that it solves so many issues later on.  Writting a good Job description may find a place midway through my list because it needs concentration and planning.  But I agree with you that most people procrastinate on this task until something makes you realize the absence of a good JD.