What does it take to be a professional photographer?

I was once told that the only difference between an amateur and a pro was that the amateur showed all of his photos and the pro only showed his best. There is actually some truth in that. But when asked that question, I usually tailor my answer to who is asking.
If one of my photography students is doing the asking, then my usual response is to explain the skills needed to do commercial work. This, after all,is where the money is made. These skills are important, as I'll explain, but photographers--especially the ones that call themselves photographic "artists"--love to get into debates that go beyond the usual amateur vs. professional dichotomy and into what constituted quality and, for that matter, what constitutes art. Such debates tend cloud rather than clarify the issue, but are no end of fun. The fact is, it is very difficult to separate the art from the profession, unless you are doing medical imaging or some such.
At any rate, the term "professional" still makes a presumption about being in the business of making photographs. That said, I've known dozens of people that have made their living off of their cameras, and yet, the work they were producing required didn't more than rudimentary camera and composition skills . The work they produce isn't what you'd call inspiring, but there is a definite market for what they do, and the customers seem feel that they are getting what they were expecting. What it is that that they expect to get is clear image of the subject and not much else. The photographer, for his part, expects to be called a "professional".
I must also state that most of the work I get paid for isn't that inspiring either. It isn't supposed to be inspiring because its function is to illustrate--not to invoke an emotional response. But that doesn't mean that there isn't some artistic mechanism working behind the scenes. An artistic background will improve even the simplest snap shot. Not so much for it being "artistic" as for the photographer having applied some simple rules of composition that is (or should be) drilled into every student of art.
When I show someone some snap shots I have taken at a party or some event I often get comments like "Oh, you are professional." It makes me chuckle because, on the surface, it sounds like they didn't believe that I was a professional until they actually saw the proof in the pictures. But I digress. What they are really saying is that there are qualities that distinguish snap shots taken by a professional from those taken by the casual snap shooter. This brings me to the second part of what makes a professional. Just what are those qualities and how are they achieved?
When I take such snap shots, I am drawing on all the knowledge I have accumulated about photography; about art; about people. It is about being confident and in control of the camera, the subject and the situation. It is not just how but also when to take the picture. It's called craftsmanship. If you are not a craftsman, the medium controls you instead of you controlling the medium. That is, you take all the blame when the results are bad, and deserve none of the credit when the results are good.
Even a rudimentary amount of technical skill equips you to stay within the limits of your equipment, subject and environment. Any good camera club endeavors to help its members develop those skills. There is all it takes to become a "professional". An even higher degree of such skill enables you to stretch and bend those limits. Now your are "really professional". If you have the rare gift to turn every image into emotional magic, then your a "really REALLY professional" and, I might add, an artist.
But it all starts with knowing what those limits are. Case in point, given a disposable camera I can get quite good results. I still apply my sense of what constitutes good composition to every picture I shoot. And I apply my technical knowledge to making sure I don't exceed the limits the camera imposes on me. Mine you, if I have professional equipment at my disposal, there are less limits to contend with. When you are shooting to get paid, you really do want to have professional equipment. Trust me on this. And, well, the customers get suspicious if I use anything less, even if my vintage Kodak Instamatic is perfect for the job. Even with professional equipment, there will challenges and problems to be solved. No amount of equipment is going to give me infinite freedom. But the more knowledge the photographer has of optics, chemistry, photographic materials, design, composition etc., there better equipped they are to maximize the benefit from that equipment and to get the results we require for the job.
And then there is subject matter. Obviously if you are a sports photographer you should have a knowledge of the sports you are shooting. How else can you get shots that are going to interest the fans? It goes beyond that. Sports photographers face different limitations that, say, dance photographers. Each specialist has their own knowledge set and each has their own skill set.
So, being professional requires you to be a craftsman, that is, skilled in visual design and in using your equipment and materials. It also requires you to be knowledgeable about the subject matter that you specialize in. And lastly, you need to be a troubleshooter, able to solve the myriad of problems that you will encounter on your assignments. Not all assignments are going to require you to use all of your skills. But your application of those skills should be consistent with the needs of the customer, and your knowledge base should always be on call to deal with the unexpected.