Zack Arias has done an interview with the founder/creator of Unsplash. It has some interesting points which have been touched on here such as model releases for the photos that are uploaded. The photographer is licensing the photo, but have the models given their consent to the end usage? If not, there are potentially some big issues.
For photographers I fail to see how this presents a good deal in a way that photo-sharing (getting your work seen and maybe attracting people) or stock websites (licensing work) did not. It's like merging photo-sharing and stock without giving to the photographers - and listening to the man's rationale, it seems to me that the site was created to solve issues for designers or clients without thinking of photographers. It also seems that someone is making a lot of money off of this, but not the people providing the content that the service needs to make money. The emails going round about inviting other people to join the 'community' of 'generous photographers' also make it sound a bit Ponzi/Cult-like, frankly.