For me, the most challenging part of working as an Instructional Designer is to anticipate things from the learner's point of view. Knowing your audience and writing accordingly and effectively for them is a thing that interests me most.
Boredom sets in when I am working on a technical project for too long. I enjoy developing content for the entire gamut of audience and have realized that my creative side is best utilized when I write for school children or for a project involving conversational writing.
I do get better with each project and there's always lots to learn . The fact that no two projects can be looked at and handled in the same way might be the reason of continuous learning opportunities.
A variety of projects on my platter keeps me forever hanging onto my job..:)