I think professionalism bores down to basic communication skills, be punctual, responsive and considerate.

I've recently hired 2 "developers" from the freelancer HN thread to find them utterly unprofessional, requiring constant chasing and inconsiderate of the projects needs.

Thankfully I didn't spend a significant financial amount on them, but, for anyone looking at using that thread I would seriously consider oDesk or something similar with professional validations as a valid alternative. It really is a crapshoot with little or no comeback, and the quality certainly isn't top end from my experience.