OS back to a shared subroutine library. Hardware becomes cheaper and users more sophisticated. People need to share data and information with other people. Computers become more information transfer, manipulation and storage devices rather than machines that perform arithmetic operations. Networking becomes very important, and as sharing becomes an important part of the experience so does security.
Operating systems become more sophisticated.
Rise of network. Internet is a huge popular phenomenon and drives new ways of thinking about computing. Operating system is no longer interface to the lower level machine - people structure systems to contain layers of middleware. So, a Java API or something similar may be the primary thing people need, not a set of system calls. In fact, what the operating system is may become irrelevant as long as it supports the right set of middleware.