From so many aspects, this is a giant step forward. Most importantly, it is a step forward for libraries and consumers. The only negative aspect is the 8 download limit, but without DRM it should easy for most users to sideload their books onto as many devices as they need. Pottermore's decision to use watermarking to protect Copyright is wonderful, as it allows the industry a chance to test the strength of watermarking in the best possible situation.
Finally, it also demonstrates the irrelevance of the publishing industry. Rowling doesn't need them; they need her. Even almighty Amazon had to change. Notice the purchase button from Amazon's page below:
This is the model I hope libraries will follow as it's only weakness so far is the amount of clicks many users will have to experience to get their content. But I would argue that Pottermore has also made strives to forcing the industry away from the tedious barriers to content, I think Pottermore's longterm goal and vision represents a giant leap forward.
One can only wonder how long it will be before a group of established author's decide to follow suit, set up a consortium and leave publisher's behind. This is a game changer.