I'm a bit late to the party, but I hope I may still answer the questions, for what it's worth.
Authors, have you ever experienced this phenomenon where your story seemed to take on a life of its own and you began to feel a certain attachment to the characters?Frequently. I love it when characters take over and help write the story. It usually becomes better that way. I've had the tops in my stories take over the scolding part a few times, which was immensely helpful to me.
Authors and readers alike, what exactly is a good spanking story in your opinion? Or more philosophically, what is a spanking story even?A story that has spanking in it. It can either contain the event (or multiple events) themselves, or be only described or hinted at. There are stories that have 'no actual spanking' in them that are really good. Of course, they do have to be, at least in part,
about spanking.
Whether I consider them good depends on many factors, including what I'm in the mood for when I read. If a story is very lengthy and descriptive, I tend to be disappointed when the spanking parts are quickly glossed over, although I've still enjoyed such stories on occasion when they were otherwise very good (well written, interesting story line, engaging characters etc.)
Is it a tool to get the job done quickly (if you know what I mean)? Or is it a piece of literature that you enjoy for its artistic qualities? Or maybe some combination of the two?That's one of those things that depend on my mood. I enjoy some stories for their
wank fodder quality (great term, KDPierre, hahah!), others for their literary quality. I find that I enjoy the stories from different authors for different reasons. Some are very sexy, others have amazingly realistic and lovable 'real' characters, others are fantastic over the top crazy fantasy stories which are guaranteed to make me laugh. I choose whatever I'm on the mood for. Sometimes I read out of my direct comfort zone because the topic of the story interests me or because it's written by an author I admire for their writing style, and I can still really enjoy it, even if it won't turn me on ;)
And some content I'll always avoid, even if it might be the best literary piece ever written, because it's too much of a turn-off for me. Different people, different interests.
When I write, I write whatever I have the inspiration for. I know I'll never please every reader so I won't try to do that either (not saying views and comments aren't important to me, though. I still enjoy receiving those very, very much :D). I've got a story published on this site which I knew beforehand wasn't going to be popular. I wrote it as part of a challenge, and that's what it was. Challenging and amazingly fun to write. It might not appeal to the masses, but the few comments I did get were very flattering and I cherish that.
So far I've only written short stories (I'm a little envious of those who can keep going for 30+ pages, I admit!). Some have the same characters which featured in another story, but they're stand-alone stories. I have this annoying habit of becoming very insecure at times, so if I keep a story going for too long, I become convinced it's terrible, start hating it, and delete everything.
For that reason I also can't let a finished story sit for too long before editing it. I do reread it several times, however, to try to take out as many mistakes as I can and remove redundant adjectives, break up overly lengthy sentences etc.
However long you want to let your story sit between writing and editing, it is wise to do the writing and editing at different times, because those activities require different areas of your brain. When I can't think of the right words to describe something or what I've written sounds off, I highlight it and come back to it later, to not disturb my writing flow.
Your story idea definitely sounds interesting so far, Astrosmurf, go for it!

(and sorry for my long response)