Great article by the folks from PixelResort.

Overall takeaway:

What makes pictograms so powerful is also what makes them so fragile. If you don’t recognize the image being depicted, the convention being communicated or the context it’s being presented in, the icon leaves you very little in terms of contextual hints. All it’s fidelity and details have been shaved away to make it streamlined to deliver a certain message. If the message fails, you’re pretty much screwed.