Gnome 1.4 was born because there was need for a transition phase between Gnome 1.0 and Gnome 2.0. We needed to develop a lot of new libraries but we also needed to make sure that these libraries would be heavily tested and used before being set in stone.
Thus, the 1.4 release contained only library additions: no feature was removed from the existing codebase and two major applications, Nautilus and Evolution, were developed with these new libraries. The diagram below shows the additions to the development platform.
The most important additions are briefly described below:
gnome-vfs provides an abstract UNIX file API: file locations are specified using urls. It alo contains the gnome mime database.
gdk-pixbuf replaces imlib: this marks finaly the end of imlib's suckyness. It provides a trivial API for image loading/resizing/displaying.
OAF replaces libgnorba: its API is similar but incompatible, its configuration files are now XML and it does not need an X server to run anymore.
GConf replaces gnome-config: it provides change notification of the configuration properties it manages. It can also have different database backends.
bonobo is one of the major additions to this release: it provides a way to create re-usable graphic controls. A la OLE/ActiveX :)