It makes me happy knowing how the story is going to turn out beforehand so I'll know exactly what I'm doing when I'm writing (that way I can add foreshadowing, symbolism, irony, etc). In fact, that's how I always write- I have to let a story stew in my mind for a while until I've got everything planned. For example, my original novel that I've been working on? I have all three sequels planned out already, plus some extra stories after the end that go up to where my characters die of old age. The only things still unclear are some details leading up to the climax.
However, when I think about it, one reason I enjoyed writing that first chapter of my fanfic is that I was doing it with only some vague ideas in mind. It had been a long time since I'd written something unplanned, and it felt refreshing in a way. Of course, now I'll be able to continue writing and enjoying it in a different way, because I think half the fun is seeing my ideas materialize just like I'd imagined them in my head, and then getting to share it with other people, and there's always a chance that something could turn out differently from how I'd planned and surprise me...but it still feels like I've lost that sense of unpredictability.
So, for the writers on my flist- how do you prefer to write usually? Planned or unplanned?
EDIT: I think I need a "JUST AS PLANNED" icon...