The most important thing we can establish before entertaining any discussion of a professional play is a clear definition. Many people will have different definitions as to what a professional player is. These definitions are often influenced by the particular field, goals, and any other specific criteria. I tend to use one universal collection of criteria. The criteria is that professionals possess the ability to perform a task that services a demand and with a declaration of such in occupation. In other words not only are they skilled, their skill is more than a casual interest.
Now that we're all settled in lets get to the meat and potatoes of our topic.
The semi-professional is often mistaken with a professional because for the most part the ability and skill sets are almost on par. So, if that's the case then why is there even a difference? Well, there are 2 things professionals generally possess as indicated by the very definition. The first is a unique or uncommon skill (not talent) that they can leverage to succeed their goal. The ability to adapt to a situation.
If you have read Jeremy Silman's work then you are aware of the philosophy that if a person possesses a certain imbalance then they must make use of it. Making use of a very distinct advantage is the easiest way to stay ahead of competition. This is not specific to chess. Businesses survive because they can provide a needed service that other companies cannot match. The advantage may be minimal or abstract, but if it is properly nourished it can provide an un-matchable leverage.
Adaptation is something that trends with experience and discipline. The importance of adaption is the more visible difference between the 2 player calibers. Generally speaking the professional will win more games because of his ability to adapt. Game 1 will often be close. However the following games will favor the professional as he can for lack of a better term... download and predict his opponents as well as the situation off of a few previous encounters, and very quickly. Hence, professionals will frequently adapt to his bracket to meet his goal. Like the company with the unique service, in order to actually stay in business longer than a year one has to adapt and modify their service so it satisfies its consumers. When they want the unique features as well as their ideal requests (ie. interface, compatibility etc. ) the company has to be able to deliver. That's where money and professionalism meets.
The reason why I emphasis these 2 factors is because in almost any professional will require these 2 at some point. Lacking in one of the 2 areas will result into an eventual breakdown. This doesn't stop at performance. Public Relation, attitude, mental longevity will benefit from these motifs as well. Fortunately like anything else you can practice these motifs in anything you do. Practice adapting to a situation, and practice cultivating your unique expertise. It will take a vast amount of time and training but the reward is of course well worth it.