I read somewhere [within a degree or two of HN] the observation that nobody has ever been able to earn expected returns from a photo-sharing site, where 'expected returns' are in the context of 'web scale.' That's not to say that at 'life-style business' size there's not a reasonable chance of success and by that I don't mean to imply that there's anything wrong with a lifestyle business so long as not attracting venture capital isn't seen as being wrong.
So my feedback is that the most important aspect of a stock photo site that wants to become a billion dollar business, would not be about site mechanics and user experience, but about the business model in the B2B space and that relates to how it is better than free or bulk at the low end and better than exclusive images from really really good photographers at the high end.
Good luck.