First off, let me say that the fact that you're writing me a story means I already love you. Please don't feel stressed, as long as you enjoy writing whatever you write then I'm sure I'll love reading it.
Things I like in fic: first times, happy endings, angst with happy endings, schmoop with happy endings (are you sensing a pattern?), obliviousness, pining, slow reveals and friendship. I have a thing for seeing how relationships are viewed by people outside of the pairing, and I like people who express their love by what they do not what they say.
Things I don't like in fic: unhappy endings, death and non-con. I also don't like humiliation or girlfriend-bashing (not that I think you'd do that!) and I have an embarrassment squick a mile wide.
I love AUs, LOVE THEM, and I'm not so keen on introspective character pieces.
I'm fine with porn or no porn, whatever you're more comfortable with. I'd like slash but I'd be happy with gen.
Fandom specific things:
Smoke Series - Tanya Huff (Lee Nicholas; Tony Huff)
Really I'd love anything, but especially something post book 3 when they're an established couple. Lee coming out (either to the world or to his family or just to the Darkest Night guys) would be particularly great, as would Tony meeting Lee's family.
Demon's Lexicon - Sarah Rees Brennan (Seb McFarlane; Jamie Crawford)
I'd particularly like something set between books 2 and 3 or something set after the series concludes. Jamie and Seb as magical boyfriends in love would be wonderful - how do they make things work for them? What do they do now? Anything. Oh and I love Nick and Jamie's friendship and Nick and Seb's antagonism so anything that worked that in would also rock.
Young Avengers (Teddy Altman; Billy Kaplan)
I've just discovered Young Avengers and I LOVE IT. So honestly anything you write me in this fandom for this pairing will fill be with joy. I'd be happy with a story that focuses on how they're just kids, doing regular kid things when not saving the world, but I'd be just as happy with a superhero story.
Whatever you end up writing, thank you!