Documents are what we work with all the time while working on computers. We even work on documents in games (the save files). So why are we switching applications all the time... Why are we not switching documents?
Imagine if you can, a blank document (a blank word document is a good start). Instead of modifying the document in word though, you add word elements to the document and use those elements to create normal word document content. Applications work on these documents by either adding elements to the document, or just working on the atoms already in the document (bitmap docs). Add a Quark xPress element to the document, and gain the ability to make really good image boxes and stuff. At the very least make layout easier.
In this system, there are only a few types of file, so any application should be designed to work with at least one of them. The types i can think of are: finite page (A4 / web), 2D infinite page (line art), 3D infinite page (normal 3D package), 2D bitmap (obvious), 3D bitmap (voxels). Other pages might be looping pages (auto tiling), cell based (spreadsheets 2d/3d, even 4d), and database (tables).
So all applications are designed to work with these data formats. All designed to work as tools upon this clay.
Sounds good to me. (and i think i sounded good to Steve Jobs (or whatever that Apple guy was called) too... i think there was something called OpenDoc that was similar, but more like OLE.