Hi Cortex
Not that I'm an expert by any means, but judging by my relationship with my own supervisor...
Someone who has some interesting ideas, who has some get up and go, who has investigated the field at least a little, who has some concrete ideas they wish to pursue and an idea of what's involved in pursuing them, who is able to communicate these things and who has a hope of making themselves understood/useful in an academic setting. On top of all that... personable, knows what I'm doing as their potential supervisor, can see how their interests link with mine, has ideas about how we can work together. I think, though, that the most important thing is being able to communicate what you want to get out of PhD study and why you think that supervisor can help you get there.