What matters to me most is that they understand what poly is about, and that my existing relationships are important, and that they leave me enough time for them and respect them.
After that, whether they are poly or mono, in other relationships or not, it doesn't matter that much. I would say my ideal situation would be either someone who is poly and has someone else, or if they're single (whether mono or poly) someone who has a very full job (time consuming job, for instance, or a very full social life).
The point is that whether they're dating me or my partner, I don't want them to feel neglected, I don't want either of us to feel like we /have/ to spend time with them. It shouldn't become some kind of chore because they're "needy" or whatever, it should be something positive to look forward to.
I don't really follow the primary/secondary model, so if they have other relationships, I'm not going to wonder where exactly they rank. And as I said, it doesn't need to be a relationship, it can be a hobby or something, it's just important for me that they have other things in their life so that even if we can't see each other as often as we might want, they won't end up lonely and bored.