Hiding, showing, and loading content across viewports
One of the commonly touted maxims regarding responsive web design is that if you don't have something on the screen in smaller viewports, you shouldn't have it there in larger ones either.
This means users should be able to accomplish all the same goals (such as buy a product, read an article, or accomplish an interface task) at every viewport size. This is common sense. After all, as users ourselves, we've all felt the frustration of going to a website to accomplish a goal and being unable to, because we're using a smaller screen.
This also means that as screen real estate is more plentiful, we shouldn't feel compelled to add extra things just to fill the space (widgets, adverts, or links, for example). If the user could live without those extras on smaller screen sizes, they'll manage just fine on bigger ones.
In broad terms, I think the preceding maxim is sound advice...