Skills And Knowledge

Good tone and technique on your instrument are obvious prerequisites to playing professionally.

There are other things that can make you more marketable and keep you in the game.
Here’s a partial list:

Strong sight-reading
Knowledge of a variety of musical styles
Improvisation
Transposition at sight
Ability to read chord symbols
Ability to play from a lead sheet / fake book
Playing by ear
Singing background
Creativity
Biz Sense (including Ethics and Etiquette)
Salesmanship
Thick Skin

All of these need not be resident in a musician in order to play professionally.  As I stated, these are the kind of skills and knowledge that might make you more desirable to contractors, agents, music directors, etc…
Do you have any to add to the list?