1) Listening to each other. They should sound like one voice.
2) Keeping pitch.
3) Clear and crisp enunciation. Theology is being imparted via the lyrics.
4) Watches their phrasing and breaths.
5) Watches their director.
6) Shows up to regular practices.
I'm in my church choir (a cappella).
ETA- And you must have true tenors that aren't really baritones- and a fantastic base section!