Smartphones are funny. They are, first and foremost, tools for communication. Their primary purpose is to connect me with other people, be it through a phone call, messaging, or online in any one of a thousand other forms. Without exception, every smartphone does those things well. Those core competencies being nailed by everybody in the industry means that manufacturers have to differentiate in other ways.
That differentiation comes in the forms of software and hardware. The user interface is a complicated beast to differentiate. It still has to be functional and efficient (or at least not frustratingly inefficient), it should be discoverable and yet attractive. And it still has to fit into the trends of modern design, lest it appear dated in comparison to its counterparts (hello, BlackBerry 10).
We saw that evolution taking place with the announcement of the Samsung Galaxy S5. The latest Samsung flagship smartphone is faster and bigger and generally better all around than its predecessors, but even as a leader in the marketplace Samsung still had to bow to the demands of contemporary design.