There are two ways to make the connection between the modified material, and an interested person. One is, at the time of modification, to trace the interested parties. The other is, at some later time, for a daemon program (or a browser) to make a search for new things of interest to a given reader.
This is an essential feature. I suspect that a mixture of the two techniques might be necessary. Efficient dating of nodes, and date-based searches provided by the server, could make it easier for the browser to find interesting things which are new. It should also be possible to create a mailing list of people interested in a given topic, and use it to mail announcements of change.
This requirement is addressed by the "Interested" relationship of HTML, along with the POST method of HTTP for a generic notification semantics.