(Can this be a meme? Because I'd love to read other people's thoughts.)
Spike/Buffy: I had a galvanic reaction to S/B when it appeared in canon, and it was as if my fannishness about the show had been smoldering for years and finally blazed up. Before S/B I didn't read any BtVS fan-fiction and didn't have any interest in writing in the Jossverse. I also didn't consider it a slashy fandom. I think I got more into S/X with S/B as a kind of gateway drug. I always found S/B slashy and wrote about that a few times.
I loved--love--how Spike is this dark horse who in the normal way of things doesn't belong with Buffy at all. To me, Buffy/Angel was the story of a young girl swept away by a brooding hero with a secret; a very traditional romantic paradigm. After Angel came back from Hell, the relationship was secret for a while, but that was for special reasons. Spike/Buffy, though--that always had to be secret. Lack of soul puts Spike beyond the pale, makes romance taboo and eroticism almost grotesque from a "normal" perspective. So I get that transgressive vibe.
I think there's a trend in some circles to peel off the transgressive label from slash (at least slash in its meta sense, as a genre that people choose to write in) but I still get a "dirty and secret and wrong" feeling off pairings like S/B. Which I like, because I then have someplace to drive the relationship to: toward normalcy and acceptance and openness. Reaching that resolution seems to be the raison d'etre of most of my pairings. I don't know if the stories I write support this, but in the romantic template wired into my brain, the crucial element is always the reaction of other people when they find out about The Big Secret. (Most often: The Big Gay Secret.)
Dawn/Giles: Ha ha! You thought I was serious...um, okay. The thing is I actually thought about putting D/G in noir at one point. I wanted to set Dawn up in an emotional relationship, and leaned toward making it someone unexpected, within the core group of characters. I abandoned this not so much because of any personal ick factor but because I thought readers would reject it. But if I'd written it, it would probably would have played out as a Dawn crush on Giles, that he didn't reciprocate deeply, though he might have had some moments of temptation. This really doesn't belong in this list--it's not a pairing I love--but when I think about it, it still interests me. Because I can see Dawn being jealous of Buffy's strong emotional ties with all the men in her life, and wanting a relationship of her own. I can see her going for a father figure too, especially if Giles started paying more attention to her for some reason, and it coincided with a Buffy-Dawn rift.
Spike/Xander: In the Pre-chippian Era, this didn't really ring my bell. And I think it actually took some meta to fire my imagination--when I heard that Joss was at first undecided about whether to give NB or AH the gay arc in season 4, I thought about it and realized how perfect a fit that would have been. I mean, Xander was always the gayer one. Maybe Joss's indecision is just an apocryphal anecdote, but I kind of mourn the distribution of storylines that became canon. I think Willow's character had a rich issue to work with already without the lesbianism--the effect of magic on her personal growth. Xander had less material to work with; as The Zeppo pointed out, he had no "thing." I think that was pretty clearly a writers-room meta discussion that turned into an episode. And they did a great job of showing that a stronger, more subtle character existed behind the facetious façade, and revisited this idea in later episodes, as when they played his matured acceptance of himself off Dawn's angst in "Potential." ("And I'm the guy who fixes the windows." Etc.) But to be honest, he really didn't have a thing. And while I actually liked his aimlessness (and Giles's) in season four, it got kind of old and lame in season five, where Anya for lack of anything better became his "thing."
With most slash pairings, nonslashers usually have a hard time seeing even the remotest possibility of a canon-based relationship. And that's true for S/X too. But I think the ambiguity of Xander's character in early seasons, and the way that Mutant Enemy handled Willow (it's rare to let characters broaden their sexual orientation), lays the groundwork for plausible Xandery extrapolation. I think that within a handful of episodes they could have convinced viewers that, say, Xander had a dirty-secret-wrong lech for Spike's pale bod. He wouldn't have said anything, but Spike would have eventually picked up on it and used it to needle him, but that doesn't rule out romance. Really, you could take that kind of tension anywhere. Let it accelerate over a cliff and into crashing flames, or turn the wheel in a new and unexpected direction. Because, let's face it, Spike's a needy and opportunistic bitch.
Xander/Anya: I loved Xander's declaration of love to Anya in "Into the Woods." I thought it was wonderfully beautiful and romantic (and strangely gay), even if not at all credible. I mean, even then, I didn't get them. But during their duet in OMWF, I heard an audible "click" as their relationship-of-canonical-convenience finally started to make a little more sense. And then I discovered that I rather liked writing Anya, and it was fun to let her unbalance Xander with banter and eccentricity.
But BtVS finale aside, Xander/Anya feels like unfinished business to me, because I still can't reconcile everything. I can see Xander taking Anya on as a girlfriend when no one else was available, and getting accustomed to her. But ME never fully fleshed Anya out and that made it harder to understand Xander's sustained interest. I still don't know what it meant for Anya to transition from demon to human, and she was mostly a bundle of character traits that never fully gelled. All facets, no real depth, because the facets are always sparkling and distracting, keeping you from peering deeper where inclusions might lie.
(I like that metaphor because inclusions can be dark spots, gritty mineral deposits, growth lines, tiny feathering cracks, or cleavage--"A cleavage has the potential to split the diamond apart along its length if it is hit at the correct angle.")
So, on the surface, X/A does make sense: he gets involved with her in a sort of casual way and lets inertia and her strength of personality carry him along without much protest; they move in together, he makes the obligatory declaration of love and then marriage proposal, but eventually chickens out of commitment. But it's more like the outline of a relationship than an actual relationship. I mean, all that snarky Willow-Anya tension is often more interesting to watch than Xander/Anya.
But, um, yes. They're still interesting and can be made fun with a little effort.
Riley/Spike: It seems so reasonable in my head. They're like dog and cat. And on the Hellmouth, why not? ("Human sacrifice, dogs and cats living together--mass hysteria!") Plus if you have a big guy/little guy kink like I do...yes. Their common Buffy yen works for me as an emotional conduit. That's a pretty classic slash or threesome bond, I think.
Then there's all the manly chest-bumping and bottle-sharing in "Into the Woods."
Plus I just think it's really sweet when Riley cups the side of Spike's face in one hand and runs his thumb over a cheekbone and smiles at him gently...I can't remember what episode that was in, but it always makes me happy.
Spike/Giles: I think it was the Career Change series that got me invested in this one, even though it's kind of a dark and wicked twist on the relationship. It's hard for me to work with Giles though, in terms of romance; he's so sharply edged that it's hard to see him softening into a relationship, especially after Jenny's death. I can't make him work with Xander. It's easier to imagine him with Willow or Buffy, but just barely. He pretty clearly looks upon all of them as children, on a very basic level. Giles/Oz is much easier to buy; it's hard to pin-point why, but it feels like the edges of their characters would mesh naturally, like the teeth in two cogs turning inward toward one another. glossing always nails this.
Right. Anyway. I was talking about Spike/Giles.
And they lived happily ever after!
Because I am sleepy and out of words.