A professional:

has his act together when it's time to start the show

pays attention to pacing and variety in the act

stays sober

is courteous to those who donated time and money so he could play

understands sound equipment

takes good care of his voice

if a singer, has the good luck not to be plagued by sinus problems, etc

has stage presence

doesn't dress in clothes suitable for turning the compost

observes audience response

(feel free to substitute "her" for "his" in the above)