One thing that surprised me about the review was the comment about how the story isn't labeled as AU but the reviewer considers it to be so. This got me thinking about the definition of "AU" and how I rarely use it. I suspect I use the word differently than the reviewer does.
"Thusia" is definitely an AU to me, because I make drastic changes from canon about what the Watcher's Council is and the religious history of the world where the story is taking place. "Dust" isn't. To me, at least. I seem to reserve the word for big shifts from canonical setting: everybody's human, or Giles never went back to the Council, or Buffy never moved to Sunnydale, or the Watchers all worship Apollo. Worlds without shrimp kinds of changes, the way the Wishverse is in canon. I think the Buffy fandom in general uses the word this way and other fandoms might consider the AU tipping point to occur sooner.
There are some stories I write that could nestle right into canon without change, but most of my output would be AU by the reviewer's definition, if I understand it. The fun is all in cluing the reader in exactly how it wanders away. One of the tricks to writing good fanfiction, for me, is establishing for the reader the story's ground rules as soon as possible. When in canon is it set? Where does it differ from canon right from the outset (if at all)? What are the key plot points the reader has to know in advance? Which ones can be revealed to good effect as the story progresses? This can be done in author's notes, but I consider that cheating. The story has to contain all the important clues in the text. At least in the version of the fanficcy game that I'm playing.
The other thing I was doing with "Dust" was remixing aspects of canonical season 6 and 7. There are a number of plot points from those seasons that I borrow and use differently, or give very different outcomes. Willow's magic addiction. Buffy's potential attack on Anya-the-demon. Xander breaking up with Anya. Buffy & Spike's dysfunctional season 6 relationship. So yeah, I want you to think about canon as you read and get something out of the contrasts of how things worked out in the story versus how they worked out in canon. But then, I think fanfiction always gets energy from that comparison going on inside the reader. This story just trades on it more than I usually do.
This might be one reason why I never get the idea of reading fic without knowing the canon. I know lots of people do, but it always confuses me when I try it. Who are these people? What's the joke here? Am I supposed to know that incident or is the writer dropping a clue for later exploration in this story? I feel I lose a lot of the pleasure in fanfic when I don't know how canon is being tweaked around, because I'm losing what makes it fanfic instead of origfic.
End of spoileriffic digression.
Random: How Pinboard weathered the Delicious exodus. A successful scaling experience. I've been pleased as punch by the service as a back end for giles_watchers. Also I was pleased by the helpful & friendly inquiry on Twitter about how my migration was going. I didn't need support (yay well-done API!) but they offered it anyway in the middle of that apocalypse.