This topic feels like it requires a few thousand words to do it justice, and I don't even know what most of those words would be. Nonetheless, off we go.
I have never had a lot of ships I feel strongly enough about to write about and read fanfic for. In fact, in terms of ships about which I am actively fannish, the list is pretty much Spike/Buffy, Faith/Giles, Meg/Castiel, Sam/Amelia, Gen/Jared, Jared/Jensen, Psylocke/Fantomex and Psylocke/Fantomex/Cluster (from X-men/Uncanny X-Force). If we also talk about fandoms I don't actively participate in, we could add Mary/Lavinia from Downton Abbey, and... I'm sure there are a couple more, but I can't think of them. Basically, it's a special ship that gets and keeps my attention after the first memory of the canon starts to fade.
I don't know that those ships listed have much in the way of common features, either. The one big one would be that most of these ships are ships that I want to work and believe would work despite major reasons why they shouldn't. A vampire in love with the Slayer, an angel who's kind of bad sweet on a demon who's kind of good, the gloriously damaged mess that is Sam/Amelia, streetwise Faith and tweedy Giles (all the better a match for having had a rebellious punk phase himself) - none of those are easy or obvious pairings, and fic that assumes they are easy does nothing to satisfy my shipping itch. What I want are ships that are hard, that might potentially clash for reasons of personality, ethics, or just age differences, but which nonetheless are shown by canon or fic to be sustainable, even joyous relationships for the people involved. There is, to me, so much hope in that; it's part of my personal need to see things built up, rather than be torn down, although of course the former is so much harder than the latter.
(The flipside is that most of my major ships have the potential for disastrous ruin, but needless to say I find fic that goes that route to be much less satisfying. :D)
The oddballs here are the RPF ships. Jared/Gen and Jared/Jensen are like relationships between paper dolls: I expect each character to maintain certain basic characteristics from one fic to the next, but beyond that it's just faces, and sometimes not even those. IDK; RPF shipping, especially the way I do it with the werebears and the space AUs, is a peculiar and inexplicable thing.
All of that isn't to say that I don't read and even write fic involving lots and lots of other ships. SPN fandom in particular has turned me into much more of a multishipper, because all the FPF pairings I like are rarepairs and have precious little fic about them. Therefore I have personally written Sam/Jo, Sam/Amy the kitsune, Sam/Tamara, Sam/Jess, Dean/Ellen, Jo/Ruby, Ellen/Rufus, and surely some others I'm not thinking of, and I've read tons more. Presently I find myself with a weakness for Kevin slash ships, which I suspect is all
*scratches head* I think that's all I've got, re: shipping. So there you have it.
Crossposted from Dreamwidth. Comment here or there. (