An article on the Smashing Magazine website - How Much Has The Web Really Changed? discusses what has changed for the past one decade or so in web development. A lengthy but easy-to-read article.
Up until not so long ago, we used to base our designs on some rather general assumptions about screen size and input type. With the rise of devices with various screen sizes and alternative ways to interact, these assumptions have turned out to be unreliable. We need to upgrade the defaults that we use when we start designing our websites.