Abssolutely the largest challenge is this: accumulating a critical mass of content to bootstrap a change of mentality amongst leading musicians, who tend to be an extremely conservative community.
I tried to do something involving this as a startup four years ago, and content was far and away the hardest challenge. (The model involved giving away content, but selling torrent-seeds when no-one was offering content. Essentially the profit model was based around marking-up bandwidth, and using out-of-copyright music as the vehicle for it).
I spoke to lots of musicians during the course of that project, and live with musicians now. My pitch is that they should get themselves well known on the internet, and then charge large prices for their concerts and for teaching fees.
However, I've found all the musos I've spoken to over the last five years to be strongly addicted to (1) the model of producing and selling some sort of physical recording, and (2) the idea that they could one day make huge money from selling recordings, despite all the evidence pointing to what a limited future that model has. Even many amateur choristers puff themselves up with an idea that the content they produce is precious and valuable in a market (somewhere), and must therefore be protected from distribution on the internet where anyone could download it for free. (Oh the horror!!)
One contact (a well regarded instrumentalist) has only recently begun to upload content to youtube for promotional reasons. It has taken years for him to adjust to the idea that this might be a good idea. He was surprised to learn that he his name is becoming synonymous with a particular composer he performs, and adjusting to the idea that this might be a good thing, particularly when he's trying to get people along to his concerts.
Once the content is available and batched it won't be difficult to place a social media layer on top of it to give ordering, tag, recommendations, etc. The content is the key. I agree that quality is important, but once there's a critical mass, musicians will change their mindset and start to compete to get the best ratings on the internet.
We'll find that the top musicians on the websites will achieve a kind of celebrity and be in high demand for tours. This will be extremely good for "classical" music which has been decadent and moribund for decades.