I really enjoyed this one. This, combined with "For Want of a Nail" make this a record year for me to actually like two of the stories, and consider them worthy of the nom. (I still haven't heard one of them yet)
The "fanfic" label never really occurred to me. This had something to do with the face that I've never seen the movie. It seemed somewhat familiar, a horror story in the Antarctic station, and I Googled it later to find it out. But in any case, there was so much invention done for this story to give the Thing a fleshed out perspective that, even being aware of the source material, I didn't have a problem with that. The perspective was so well done that I really felt like I understood the well-meaning reasons behind the horrible things the creature does, and I was sort of rooting on this creature that could quickly overtake all of our world's organisms.
Regarding the label of "fanfic", I guess I use a more narrow definition than many of you. To me, "fanfic" is one of those terms that's always derogatory. Generally what I call "fanfic" is unofficial, probably anonymously posted to someone's blog or fan site, certainly not professionally published. "Slash" fiction or whatever you call it where someone writes about their favorite characters all having sex with each other, is generally the sort of thing when I think of "fanfic", not well-written and extremely imaginative stuff published at award winning magazines like this. To me, I find the moniker insulting, perhaps because by the broader definition I've sold fanfic as well.
I'd certainly not call it plagiarism either, as all the words are Watts's own. I assume Watts got explicit permission to publish this story, else I can see how it would be called copyright infringement.