I am extremely happy about Watson, but I need to point out some possibilities:
Imagine this scenario. John earns 100K $/year, so he gets access to Watson. Jerry earns only 30K $/year, so he can only use the open source one, which has 10% less accuracy. That would be tragic.
Another scenario: doctor X sees the Watson prediction, and has a doubt about them. He doesn't say anything for fear he could be charged with malpractice if the patient dies under his different treatment. Thus, doctor X is reduced to a yes man, and his boss is not even another man, but an artificial brain. How many doctors would dare oppose Watson ? Do we need doctors any more if all they do is rubber stamp Watson's results? What about medical students - if they know that the difference in diagnosis accuracy between a medium and a top student has been reduced to just 2-3%, why strive to learn more than the other? After all, it's the same whether you are in the top 1% or around the middle of your class.
Ok, now for a positive scenario: we get cell phones with biometric sensors (medical tricorder) and we can input our symptoms. We are trained to do that because we took a 3 month medical class while in high-school. It was compulsory. So, now we can treat 80-90% of our problems without going into a doctor's office. This is amazing especially for remote areas and places where there is no access to doctors for hundreds of miles.
Africa would be revolutionized by it. A single medical nurse with a cell phone / mobile lab could service many people.