For myself I think that I more or less look for the same qualities in a D/s partner as I would have in a vanilla situation. I will say that a willingness to explore your own desires is even more key in D/s, and also a willingness to share those desires (once trust is there, of course).

But really I don't think much changes. Do we have fun together? Do we have similar interests? Are we attracted to each other? Do our lives allow us as much time as we feel we need together? Do we trust and care about one another, or are we just hanging around for a good time? Does she tolerate my nerdy hobbies? Questions you could ask in any form of relationship.