"So. What are you looking for?" Every agent or editor answers this question thousands of times, across their career. Here's my first stab at it as an agent! (Note: I'll be updating this list regularly, as my list grows; you can search via the "Manuscript Wishlist" tag for updates.)
In picture books, I gravitate in three directions (though not necessarily all at once): sweet and gentle, wittily subversive, or outright silly. I also have a strong appreciation for well-written picture book biographies that combine a vivid sense of personality and character intertwined with the facts and historical details that make their life unique. I'm rarely, if ever, drawn to snark, or picture book manuscripts that are primarily lists or descriptions of a character's day, rather than fully-developed stories.
In all stories (but particularly MG/YA fiction), I get excited by:
A careful balance of universal elements and specific details: making any reader feel a sense of connection between their life and your story, while also presenting a highly memorable read.
Strong depictions of place, whether real or imaginary: where the richness of the setting/world-building has powerful influence on character/plot development. (I have an especial fondness for Southern voices, having grown up in Texas.)
Friendship stories: in picture books and middle grade, of course, but also in YA (since the teenaged sense of self is so often shaped by friends). "Band of misfits becomes a family" is a favorite trope of mine, as are books that might best be described as "the love story of a friendship" (rather than a romance).
Stories of heartbreak, heartache, and heart-healing. And on the opposite end of the spectrum, humor! In short, I'm looking for books (and characters!) with the power to make a reader laugh, cry, think, or feel, deeply.
Stories that are fresh, inventive, or ground-breaking in their structure, format, or plot, while also having a strong emotional core. (Think: re-tellings, genre mashups, speculative fiction, and other stories based on a compelling "what if.") I get excited when an author finds a new, utterly unexpected way of telling a seemingly-familiar tale, or builds a world that diverts from history, science, or reality as we know it, alongside of a character we can't forget. If it's never been done before but you're convinced that you're the person who can pull it off (whatever it is!), then I want to read it.
Cinematic writing: if readers are dream-casting in their heads while they read because your vibrant storytelling, sweeping settings, amusing worldplay, and vivid characterizations, it's a good thing.
Magical realism: I can't get enough of stories with worlds that appear much like our own, but that have a touch of the fantastical or extraordinary. (Note: magical realism is different than fantasy.)
Action & adventure balanced with richly-developed characters: If you can keep readers rapidly turning pages, but also make them care about who your plot is happening to, chances are you'll quickly have deeply-invested readers.
Diversity in all its facets. Our books should represent our readers. All of them.