@Christiaan—In regards to context, I think that’s simply really difficult to assume. For example, you mention a mobile user being on the go and a tablet user being on the couch. How do you know that’s the case?
I tend to think that responsive websites should be used when you simply want to make sure that all people can access your content on any device—not to present different content to different contexts. You have to assume too much.
However, future-ready content isn’t just about responsive design. If you look at what NPR is doing, their model delivers different combinations and lengths of content to different users depending on the suite of NPR product they’re using: one of their smartphone apps, NPR Music, member station website, etc. But in those circumstances, the user has told NPR they want a “special” experience by visiting or downloading one of the specialized products. Going to NPRmusic.org instead of NPR.org says “I want music content,” and logically users there get a different depth of content there. Same with downloading the NPR News iPhone app, which says “I want news.” Users’ exact context is unclear, but these things give us clear direction.
But assuming what people want based on device size alone in a responsive design, and offering different content to those who have not asked for something special, seems problematic to me.