Additional Likes and Dislikes Character: McGonagall is my OTC; give me a believable Minerva paired with anyone except Trelawney or teenagers or ghosts, and I'll be thrilled. But if you don't feel like writing Minerva, that's perfectly fine; I like many others, too, especially the Hogwarts staff and Ministry personnel. I love all the older women except Molly, Bellatrix, and Sybill. For the main pairing[s], I'm not very interested in Trio-era teens (even when grown up), the Marauders, next-gen characters, the Tonks or Weasley families (except Aunt Muriel), or most DEs (all these are fine as supporting characters, though). In general, I like to see the older characters as older characters (i.e., not as teens or 20-somethings except as background), but that preference is flexible.
Plot: I enjoy missing scenes, backstories, overheard conversations, characters observed by others (anything from casual eavesdropping to full sexual voyeurism), canon-compliance (although I also like AU as long as everyone is IC). I don't mind at all if you find believable ways for canon-dead characters to survive. You don't need elaborate explanations for their survival, unless that's what you feel like writing. I also love stories that keep the basic facts of a canon scene, but then subvert it in some way, offering a different understanding of it.
I like power issues and struggles, hurt/comfort, political dimensions, inventive magic, glimpses of the every-day magical world, first-times as well as established relationships, angst, realistic bodies and sex, BDSM, cross-gen (but not with teenagers, and if het, I prefer the woman to be the older). For this fest, I prefer stories that fulfill the stated purpose of hp_beholder --- "to bring a little love (or lust) into the lives of our favourite Harry Potter characters who are otherwise often overlooked." For me, love (or lust) fics don't require happy endings (though happy endings are fine), but they do require some sort of consensual arrangement and, at least initially, mutual desire. As for eras, I'm happy with anything from the first war (c. 1970s) to post-DH.
Overall, it's grown-up, IC characterization that matters most to me. (Still, I like there to be a little meat to a story, something beyond a character vignette or a PWP. These genres can be lovely, but they aren't usually stories. And I'm in the mood for a good story.)