Computers can mediate many things: music, or visual art, or poetry. But I'm mostly interested in stories, and storytelling, and narrative. That can be as text, as our stories have been for millennia, or the more recent visual form.
Within this topic, there's still a host of interesting subareas. Most basically, software has created a mildly new medium, of short animated, semi-interactive cartoons. Second, computers can enhance existing works of literature, by offering (semi-automated) annotations, cross references, indexes, etc. It's actually surprising that there's been so little attention spent on this, or on the closely allied question of enhancing non-fiction works, especially textbooks. Second, computers can, potentially, generate stories. It hasn't been done yet in a convincing fashion, but you can certainly imagine more sophisticated programs generating decent, if boilerplate, stories. In form, such stories would be no different than standard works of literature (in fact, would probably be indistinguishable from Hollywood summer blockbusters or sitcoms); different only in their authorship. Software might serve as an improved authoring tool for literature. Certainly, the word processor has made author's lives easier, and there's countless packages that claim to help you write screenplays. Have we really exhausted the ways in which software might help writers?
All of these are fairly interesting, but there's another topic which holds the most promise. Software could act as an intermediate between us as readers and some form of new medium of story. Hypertext, interactives, and even more exotic forms of literature could be as different from all of our existing media as writing was five thousand years ago when stories were first written down.