"To be generalised you must be comprehensively so, to be specialised you must be exclusively so."
These words of wisdom contain more truth than meets the eye, but can be illustrated quite easily by applying them to web design.
Generalised sites, like portals and search engines, have to have a comprehensive index: the idea is that the site encompasses everything.
Specialised sites, however, have to remain specialised: they have to contain all available information on their chosen subject, and possibly a few links to related subjects. As well as this, the webmaster has to ensure that he doesn't stray onto other topics, or the site will lose its specialisation.
Robert A. Heinlein said that "Specialisation is for insects," but it may be said that specialisation is also for works of reference: the science dictionary has its place as much as the enyclopedia. In the same way, portals are merely the indices of what is to be found in the volumes of specialised sites. Only in this way an the world wide web fulfill its function as the World's Greatest Encyclopedia.