Among the gripes I have about responsive web design is the issue of display ads. The most popular ad units today are a product of the desktop web experience. These are fixed sizes and entirely unfit for the mobile form factor. In keeping with most mobile metrics, mobile display advertising in the U.S. has over doubled in the last 24 months. Again, these are ads at fixed sizes, and there are many of them to suit various mobile screen dimensions and densities.
These can be nightmarish conditions for responsive design.
So here's a thought: Why not experiment with ads that are themselves responsive? They could be packaged up with their own media queries and progressively dependent assets and served to a wide range of devices.
Someone out there should try it. Just sayin.