Drupal has been a great site management system. It is called a content management system but there are some functions that would make it much more useful in that regard. I say it's a great site management system because it's great for managing a single site. If you want more than one site you need more than one instance of Drupal. There are some ways of prefixing table names and making it possible to put two instances of Drupal on a single server.
Taxonomy based RSS feeds would be a big help. It would be best to have the ability to have several different RSS feeds so people could subscribe to those feeds they are interested in. If a site owner chooses to they can allow for people to subscribe to various feeds based on taxonomy.
Sites based on taxonomy would be an even greater opportunity. This would probably be complicated to implement and would mean changing how configuration happens. Either there would be a specific taxonomy that routes nodes based on database prefix or there would be configuration kept in the database. The key would be that certain users would be able to access a separate back end and enter content that would route to one or more sites.