Attitude and aptitude.
I know of a few great players who have bad attitudes and are difficult to work with. They don't get called back for gigs, if they even get called at all. A few things to remember: be courteous, be on-time, be open to change, don't argue with the leader, say 'thank you', remain teachable, etc...
And you have to know how to play your instrument, know how to play with others and know your gear. Pro gigs = time is money. If it's a gig that required you to practice the songs beforehand, do that! Rehearsal is NOT the time to learn the songs (unless you've been given zero info prior to rehearsal). Be able to set up fast and get a good tone fast. And be stylistically diverse.
An average musician keeps themselves restricted and in a box. A pro musician sees every opportunity as a chance to expand and be fresh and expressive.