I would say something different for biblical scholar and theologian (the two are not always the same), but for biblical scholar:
Attention to detail combined with breadth of interest.
~ John M. G. Barclay, Lightfoot Professor of Divinity, The University of Durham
John is definitely right on both counts about what makes a good biblical scholar: the former without the latter leads only to missing the wood for the trees, and the latter without the former leads to a severe lack of depth and critical acumen. But what then makes a good theologian? It is a question I fear to pose, because I suspect the Orthodox tradition is on to something when it limits the genuine use of the title “theologian” to very few uniquely insightful bearers of the light of truth.