I think some of the first slash that I ever read was for Jonathan Strange & Mr Norrell because, at the time, there was so very little of any fanfic for that book that I was willing to take whatever I could get. Or, at least, I was willing to try what was out there. Most of the slash fics which I have a vague recollection of reading back then were nothing particularly remarkable either way (although the same goes for a het fic or two I also read, because that one really awesome character is just asexual), although one was dreadful and frightful on various levels. Now, there's a lot more out there, but I just haven't gotten around to it; this is partly laziness and partly that since the whole novel is so asexual (desexualized?)---even in the canon het pairings---that it just seems ridiculous. That said, I would like to see if, say, Drawlight/Lascelles could be done reasonably, or Lady Pole/Arabella Strange (there is perhaps one other pairing that could be plausible, but I think any writing for it would utterly defeat its . . . viability, or maybe just appeal).
The thing with slash, though, is that I did not at the beginning and do not now read slash for the slash. I read it because my favorite characters tend to all be male, and much of the writing for them is, therefore, prone to being slash. I therefore took up reading Hamlet/Horatio not because I really liked the idea of Hamlet/Horatio, but because if I wanted to read anything about them---indeed, pretty nearly anything at all about Horatio---then Hamlet/Horatio was what there was. Again, though, most of the fic I encountered was not especially remarkable. Of course, most of this time I was nosing around on the endearing Pit of Voles, which does not really have the best of reputations for turning out the most endearing of products.
I think, however, that the first slash I actively liked was
hyarrowen 's first Henry/Montjoy stories when I stumbled across them on the above-mentioned inauspicious Land of Numerous Fandoms. I have, for years, very much liked Branagh's movie of Henry V, and from early on I particularly noted and liked the character of Montjoy. My first reaction to the existence of those stories, then, was, `Yes, they're the two coolest characters in the play, but that really doesn't mean you have to slash them.' This was followed by a certain amount of reflection and then the concession, `They are the two coolest characters in the play,' and ultimately clicking on the links. I have not had cause to regret this, and indeed
Manning remains very much a favorite of mine, though hyarrowen's subsequent stories are indeed also excellent.
The thing is, the good writers of slash---the ones who create something substantially more than just two guys getting together and/or going at it (and I have just recently been reading a string of not-so-good ones)---do manage to capture something very intriguing of the relationship between their two characters. Such stories are not just love stories, but actually manage to somehow, at least somewhere within them, illustrate or offer some commentary on the dynamic the characters have with one another, and yes, there's the romantic angle as well, and sometimes that's an important part of the relationship (but only one part) and other times it's little more than a flavor (
`Nom de Coeur', for Casablanca, is an excellent example of this latter). Indeed, it is often more pre-slash than slash that I find most appealing. This is not, however, because I am inherently a fan of angst any more than I am of slash; it is merely that angst, or more generally character-in-situation-of-hardship, is conducive to character exploration or development.
And somewhere along the way, I have become more receptive to reading slash pairings and now even keep an eye out for potential slashiness. Even so, there are pairings that I simply do not ship, even though I can see where other people draw their evidence for them. I do not take slash in my Age of Sail (the Twenty-ninth Article of War is part of this, but not all); Holmes/Watson is a non-happening thing for me (or, at least, Doyle's Holmes and Watson, although Doyle's Holmes and Laurie R King's also clash, and I will grant that the Granada series actually is rather shippable, but King and Granada both represent certain distinct departures from Doyle); I am also emphatically not a Kirk/Spock fan (indeed, I once read a Kirk/Spock fic out of arbitrary curiosity that had marvelous characterization, quite spot on . . . until it would come to the romance, which was not by any means badly written, but at which points the story just stopped making sense for me). For all of these, granted, I knew the characters well before I was aware of any such thing as slash, and for whatever reason I refuse to retroactively accept them as slashable.
Shakespeare characters, by contrast, I am more willing to admit to the status of potentially slashable. Perhaps this is because I am already used to the idea that there exist as many different interpretations of the stories as there do productions of the plays, and thus it is not much effort at all to acknowledge the possibility of Olivia/Viola or Antonio/Sebastian despite the fact that I have known Twelfth Night (or at least the 1996 movie) forwards and backwards since (again) before I was aware of sub-textual same-sex pairings (or, perhaps, even homosexuality in general). Although I do grant the validity of such pairings as Antonio/Sebastian or Antonio/Bassanio or Mercutio/Benvolio, I don't have that much interest in them, but this is due to a general lack of interest in the characters themselves, and adding a bit of romance to their relationships doesn't go far to recommend them . . . not unless there is something worthwhile in the actual characters as well;
pargoletta's
Caro, for example, has indeed shown me appreciable aspects of Mercutio and Benvolio.
All of which is, again, to say that what I have found to be most compelling to me of slash fiction is the overall treatment and exploration of the characters---well-liked, familiar characters to whom I am much attached---rather than that one feature which gives the genre its name, although this itself is not unenjoyable. As I've explained it, I grant, this may sound rather demanding, but I do not think of it as so: I don't require character driven epic; I like just some quality time with old friends.