The current open call is for author owned properties and will follow a much more industry-standard kind of contract. That's not my department, so that's about all I know.
As far as YA goes, I'm the person to talk to, actually. But I'm only looking for series proposals right now (shared-world, not author-owned stand-alone). Hopefully, we'll be moving in the direction of author properties in the future, but we don't know when that will be. If your manuscript is something you think would be be easily transferred to an adult market, I'd go ahead and submit to the open call. If you think it could be turned into a series, even a limited one, send it to me and if I like it we can discuss how the contracts work in more detail. It's not so much that they're not as favorable as they're different, especially if you're the one who came up with the series idea. Mirrorstone's writing guidelines are here:
http://www.wizards.com/default.asp?x=books/main/mirrorstoneguidelines. You'd address the submission to me (Stacy Whitman).
EDIT: I should clarify--Mirrorstone is the YA/children's imprint, which I am associate editor for. The open call is for Wizards Books, which is the adult line, separate from my department.
