Let me be perfectly clear: nothing's wrong with writing entertainingly; I'm all for it! But it does mean that other kinds of work have a harder time surviving. Some writing is designed to give the reader a rollicking good time; some is designed to ask tough questions, to challenge us as readers and as people, and to take us on spiritual or emotional journeys. These books are not always the easiest to read, or the best-suited for breaking into easy, bite-sized pieces. But that doesn't make the writing any less necessary.
What Do You Want Your Story to Do?
If an author's goal is blurry, the story will suffer. We as readers will feel confused if the story is half-joking, half-serious, or seems to accuse us at one point and be in cahoots with us at another. You the writer must ask yourself some tough questions: do you want to push the envelope of style? Raise difficult political issues? Challenge established religious ideas? If so, you may have a very heavy, intense piece of writing ahead of you. Prepare accordingly!
What Kind of Writer Are You?
This is the difficult judgment writers must make - they must decide what they want to write. That doesn't mean crafting an elevator pitch for yourself or deciding what blurbs will be on your book jackets; it means deciding whether you want to push the reader, deconstruct things, soothe the reader and provide a balm, enliven a feeling of nostalgia and shared experience, make the reader laugh - or all of the above. Your writing will be special in one way if you focus on sharpening and honing it, but it won't be special at all if you try to do everything at once, or pander to what you think your audience wants. Too much now, there is a divide between reading as entertainment (often looked down upon) and reading as intellectual exercise (often too divorced from the pleasures of reading). If you're going to be tough on your reader, don't be apologetic about it! Don't compromise the principles of your story by throwing in cheap entertainment just because you think you need it.