The most important quality in a graduate student is playfulness. A light-hearted excitement about exploring something new.
My heart leaps when a student says, "I was thinking" or "I was wondering", and when those thoughts are far beyond the current topic of conversation. The click-clack of connections, new ones unconsidered before.
This is a student who goes to the library because they want to, not because they have to. They read books far outside their discipline. They create wild, innovative ideas as easily as they breathe.
The best part of my job is when I meet a student like this, and feel great hope about the future.