I prefer to think that responsive design is still in its relative infancy and as an industry we simply have not completed the shift in mindset that is necessary. When we work stakeholders who are not technical it's clear that they're 1,000 miles away from the mindset, and the average designer is still somewhere from 10-999 miles away. Fluid design is a big step forward and while we still don't have anything near a bible these are some example principles:

* Size elements relative to the size of the viewport, some things should be smaller on little viewports, some things should be a little bigger, the CSS units for this are still just coming together. Use formulas and percentages, not pixels.

* Think about the correct layout direction for everything on a portrait vs landscape orientation, should those items be in a row or a column? This often requires only one CSS property to change

* Hide information density behind an accordion or modal if you need to on small viewports, don't just remove the functionality for all versions of the application. When people strip away functionality, I find design is rarely the true justification -- more commonly they no longer want to pay the maintenance cost of the feature and are using a redesign as an excuse.