My philosophy for writing requests is, I try to give a core request and then surround it with a lot of possible approaches to take. This is because my favorite requests I've gotten in the past would list a bunch of possibilities, and I would find one of those fired my imagination. I don't expect you to meet all of those possibilities! The core request matters, but for the details, find an approach that works and run with it.
As for my general tastes (again, not all of these need to be accommodated in one story!): I tend to prefer case/adventure/mystery stories, but if that's too much, you can also satisfy this urge by having a personal/talky/less-plotty story with some kind of adventure going on in the background. I adore hurt/comfort, both physical and emotional, and I'm always a sucker for the trope where character A briefly thinks character B is dead. I'm mostly a slash fan, but I've got nothing against gen or het. The emotional connection between characters is usually more important than the sexual one; I'm always happy to read smut, but I think even a romantic story can be complete without detailed sex. I get wobbly for characters doing stiff-upper-lip as long as I can tell they're holding back genuine feelings. I do prefer happy endings but I can tolerate ambiguous endings as long as they aren't completely miserable. I dislike character-bashing and partner betrayal and deep dark themes without getting some light in eventually. I like AUs in medium-to-big fandoms, but usually in small fandoms I'm looking for something more basic because there's hardly anything out there at all - but you can sell me on an AU if you work at it. Not really a fan of character death unless I already got exposed to it in canon - in that case, you can leave them dead or resurrect them if you like.
I just got my assignment for this year, and it's not my very favorite nor the one I thought was most likely, but I'm beginning to get enthused. Gotta bone up on the canon again, that will be SUCH a trial...
Now, for whoever gets to write MY story: usually I try to be non-specific, but in the how-to-write-your-request posts it seemed like more people complained about not enough guidance rather than too much guidance. So I'll try to be specific but give you lots of different options to choose from.
My general preferences: Most of all I like plot (adventure, case, battle, etc). If you don't have time or story room to write a whole plot, you can fool me by writing a vignette that implies plot happening all around it, like a moment in medias res. I like slash but I don't insist on it -- you can satisfy those urges with non-explicit slashiness, or strong friendship, or hurt/comfort. I mostly just want to see the connection between the characters, and if there are het relationships involved I don't want those to be the focus of the story. I usually like to see a style that matches well with canon or with the author's writing style and language, since all my requests this year were bookfic. But if you want to experiment with a different style, go for it -- just make it conscious and deliberate and consistent. I like strong characterization and being able to hear the characters' voices in their dialogue (I know that's tough with bookfic). I like either angst or schmoop in moderation, with a general preference for happy endings. I'm always willing to read explicit smut (and if you can write smut in the author's style, WOW!), but it isn't necessary to make me happy.
As for specific requests: In all of the first three requests I just really like the connection between the characters in canon and want to see more of it. For the Nancy Springer, I'm okay with reading incest but have never been able to write it, so I threw out a request to get someone else to write it for me -- there should be lots of potential for angst there. Same thing with the master/apprentice dynamic for Pamela Dean -- I can't write that but like to read it if you're game to try, or you can fall back on friendship and distract me with pretty language and Shakespeare quotes. With the Martha Wells I'm envisioning a Sherlock Holmes type mystery but it doesn't have to go that way -- maybe first meeting or first kiss or angsty rejection of one-sided crush or something.
The Bujold request is probably the hardest because it's so vague, and it's the only one that isn't character-driven. Here's where I'm coming from: I like the world Bujold built, but as far as the main characters go I'm happy with what we're getting in the canon and don't feel a drive to see more. So I'm looking for something different -- another part of the world or the history, or an expansion of some secondary characters (I generally like the Lakewalkers better than the farmers), or something like that. I have found myself toying with putting people from many of my favorite fandoms into that world and seeing how they would be different, how it might work if ATA gene=groundsense or Sentinel senses=groundsense or something like that. I'm intrigued by how little we know of the past world with the sorcerer-lords and maybe the gods weren't absent at that time, and how did that work? So there are a lot of different approaches you could take with this request, and I'm hoping that one of my suggestions will gel for you. I still have the general preference for something that's centered around a character or a couple, whether original or canon-secondary or transplanted from another fandom. I still have a general preference for m/m slash, but also willing to read m/f or f/f for this request.
I hope that's enough for you to work with! Just don't worry too much -- remember, I love these worlds and characters and will be happy to see anything written about them!