On mobile context and responsive web design
The fact that the user has a small screen in their hand is one thing – the fact that it is in their hand at all is another. The fact that the user may be walking, driving, or lounging is yet another. In fact, it’s quite likely that they really deserve different content and services altogether – or, at least, a differently prioritized version of the default desktop experience.
Emphasis mine. That’s possibly the most important paragraph from James Pearce’s post Not a mobile web, merely a 320px-wide one.
In other words, a “responsive” web site design works best when the site’s content has been prioritized.