Windows 8. Chromebook Pixel. Ubuntu Phone. These devices shatter another consensual hallucination that we web developers have bought into: mobile = touch and desktop = keyboard and mouse.
We have tablets with keyboards; laptops that become tablets; laptops with touch screens; phones with physical keyboards; and even phones that become desktop computers. Not to mention new forms of input like cameras, voice control and sensors.
One of the core things that responsive design has taught us is that we have to be comfortable with the ambiguity of not knowing what the size of our canvas is going to be. Input has that same ambiguity. It is transient. It is unknowable. Reconciling that understanding from a design and implementation perspective is going to be as big a challenge if not bigger than the one we faced coming to grips with responsive design.
We've learned how to respond to screen size. Our next challenge is learning how to adapt to different forms of input.