At the risk of semi-disagreeing with Professor Zero's "A Heretical Post," I have to qualify what she says when she says "Writing is fun. Publishing is easy."
She's right about the writing books that moan about writing and about the Frail Souls who put their hands to their foreheads when telling you how busy they've been. Maybe they have been busy, but they haven't been busy shingling roofs when the temperature is 104 degrees, nor have I, so let's not kid ourselves about the kind of hard work we do. So when is writing fun for me? (Your mileage may vary.)
1. Writing is fun when you're in the flow of it. Unfortunately, that "flow" experience sometimes gets spent on other things, like a piece of grad student writing I was commenting on the other day. My brain was pumping, I was making suggestions that will make the writing better, and I was enjoying that "flow" experience through commenting--but it didn't contribute one word to my own writing.
2. Writing is easy, but starting writing, and thinking about what you want to write, is hard. Over the weekend, when my colleagues were variously skiing, hiking, enjoying conferences, and visiting tropical places, I sat stubbornly in front of the computer monitor while trying not to bang my head on the desk while I tried to think through some concepts for a deadline-driven proposal. I'd like to think that doing worthy things like going to the gym or washing the floor or finishing an article review would help by breaking up the process, but all that does is say to my brain, "Why, you've worked a lot already today, haven't you? I guess you're all done."
From various comments, I'm guessing that my colleagues don't go through this long, slow process of gearing up to write. I've asked them about how they get started writing or if they get stuck a few times, and they look at me as if I've grown two heads. They're more like the people Boice describes in Professors as Writers, or the ones that Silvia describes in How to Write a Lot, who never experience anxiety because they know that they will write every day at a set time, without any of that nasty agony about ideas.
3. Writing is fun once you've finished a piece. Have you ever noticed how right after you finish something, you sort of love it for at least a few minutes? Everything, however minor, gets some admiration right after I finish it, from a politely cranky letter to a state politician to an article that gave me grief. Of course, two hours later I am dissatisfied with it again, but in that immediate glow of relief over finishing something, I'm happy with it and with myself for getting it done.
4. Writing is fun when you see your work in print. Again, like the "I've finished it!" afterglow, this doesn't last. You get the book or journal, you start to read, and soon you notice a sentence that you would totally revise if you had the article back again. But that's the nature of creating anything, isn't it? I've heard of famous directors who had to be barred from the projection room even after their films were released, because they'd try to go in and recut things.
So yes, writing is (or can be) fun, else why be an academic or keep a blog? But if it were easy, I'd be a few thousand words into the next project by now instead of trying to procrastinate by writing a blog post.