What makes a good TA?

Above-average knowledge in the subject matter.

A good work ethic.

Communication skills.

Service-orientation.

A willingness to stretch.

We in the department were discussing possible TAs.  There were several in particular--let's just lump them all into a single composite, X--who were disqualified because they were very smart but then lacked that the fourth and fifth criteria.  X's agenda didn't coincide with that of the department.  X was much too preoccupied with other things--extracurriculars and what not.  Furthermore, to paraphrase Yoda, we detected great fear in X, most notably a fear to embark on something that would require going beyond comfort zones.

Though I admit that, upon reflection, I find X a little self-aborbed and user-friendly, X is not evil.  X would just not make a good TA.