I finished Spirit a while ago and just barely sent it off to Writers of the Future. Spirit was an interesting test. I wanted to write something different; short (since I mostly write novels), in a POV I normally don't use (first person) and contemporary.
Of course it turned into fantasy, but I managed 1st person and the voice just popped right off the page. I had some serious concerns about being able to maintain a voice that's so different from what I usually write, but I think I did OK.
Since I started Spirit I've written a couple more shorts (mostly for contests that require under 500 words) and it's quite interesting.
Fitting a whole story into 500 words (or in the case of Spirit, 16,000) takes a different set of skills than writing a novel. It was actually harder to write 16,000 words than 500 and still get all the story elements in there.
When I write I think maybe five minutes ahead, which is perfect for a short story. With Spirit I had to try to be a pantser but stay within the contest boundaries. I still doubt my ability to maintain a voice like that through an entire novel. It's just too foreign. Once I got through the first half, I had to be constantly thinking about the voice and how it developed in order to maintain it.
Part of the problem is that I tried to stick sub-plots in there as I would in a novel, and there's just not room for them. Then I tried to end it too soon and wrapped it up too quickly. Fixing that meant going through every stage of a story and making sure it was all in there. I removed a couple sub-plots, although one of them got tangled with the climax and had to stay. :)
The hardest part of the whole process was hitting "send."