- it’s built with URL support in mind, you need to do quite some work to not use URLs. I think this will be quite important for 500px like site, because when you browse through the photos, you want your back button working correctly and you want to be able to link to any of the photos
- it seems to me that content on such site will be highly cache’able, ie. it won’t change that much between different visits. Sure, you may implement “like” functionality, comments and other interactive parts, but it’s often the case that most people view the content rather than to create it. This means that it may be easier to make it work fast
There are probably other good reasons to use Ember, you should do your own research and compare it to alternatives anyway.