So masteries now have much more deeper specialization for every champion

It's also much, much easier to adjust a page. Rather than needing to reset a page, or pull a lot of points out, to swap stuff round, you can just click on the alternative to automatically swap points from one option in a tier to another. E.g. if you've got 18 points in Cunning and want to swap from one Tier 1 Keystone to the other one you don't have to pull all the subsequent points out of the tree, but can just toggle to the other Tier 1 Keystone with a click.