I just got yelled at in a Comment by Brad on a recent post. In my answer I pointed him to an older post on GSD, because I am serious about the mental habits and growth of my team in general. In support of GSD, a removal of any "language tunnel vision" is something of a necessity.
When we're recruiting, we look for a willingness to try new things more than what people actually know. We're doing that now, so I've been thinking about this a lot.
The GSD links back to another old post, in which I state, equally seriously, that consistent and disciplined use of a developer tool or tools is no guarantee of productivity.
But what is? Developers have to have logical minds, sure. What else, as personal characteristics?
an ability to work independently, and think for yourself, balanced by
the energy to research when you can't find the answer alone
a committment to excellence in the result
an interest in the implications of a requirement, not just a superficial resolution
implicit trust that being taught to fish, rather than giving dinner, is a valuable gift, and shows that the giver trusts you
... what else? What's on your list?