What's missing on this planet is a site that would connect doctors and patients online all over the world.
There is a ton of crappy medical Q&A sites and they all suck.
The situation is the same as before StackOverflow - a ton of crappy programming Q&A sites and everyone thinking that's how you do it.
There is a Health section on StackOverflow but they don't allow personal problems, plus this site needs to be different: the user should be able to enter his medical history in a unified and easy way.
For example, there should be a drug database, so as soon as he starts typing what drug he's currently taking, it will auto-complete it. Symptoms should be unified like tags. Some sort of timeline to show clearly what's going on with this patient. A way to upload documents obtained from hospitals, like x-rays, blood tests, etc. Private chats with doctors as well as publicly visible responses.
There needs to be a good rating system for doctors and their replies. And a way to keep the signal-to-noise ratio high, same as at StackOverflow.
Bitcoin can be integrated for more incentives and even something like Watsi. Insurance can be integrated too.
Whoever manages to pull this idea off will make a huge impact on the world, because the demand for such service is insane!