well, to me... seeing that I am one... I aspire to be professional in a lot of ways...
but aboveall... no pun intended, originality and giving credit to others work, not taking it for your own, contributing new knowledge to the industry... so many 3D artists in the gaming world and rendering world use other peoples models, and then take credit for it... normally from one program to another... if you frequented 3DMax forums you could use their models, convert them, paint them, and so on... and then take credit for it, and the chances of anyone knowing are slim... this happened to me while building a racing simulator... I Can't say who I worked for, I had to sign NDA's but EA's SIM Racing and rFactor we're involved... and there were lawsuits flying... all over who's model was whose... and how they went about proving it was interesting...
anyway I digress... I think a Pro is someone who brings something to the table...