Scales. Chords -- traids, sevenths, extended are nice too.
Ability to keep a steady beat with a group.
Ability to improvise a little, and stay with a group.
Ability to prep music or make something up from a leadsheet.
Ability to hear a line and know what the pitches are and what harmonies go (or could go) with it.
Ability to transpose in case a vocalist wants to change the key on the spot.