A Digital library (DL) is an approach to storing, and cataloguing information in the digital age, and many CMS projects have been attempting to achieve, whether they call them digital libraries or not. However a lot of packages make the integration very disjointed. Lets take an example:
A website devoted to building boats. There will be articles, photos, links, files, and forums.
However with a lot of existing CMS software each of these object types will end up with their own set of categories, often only allowing one category. This is the first breaking point of the DL idea. In a DL you should be able to take a cut along and category, keyword, etc combination and get a list of all the content in the collection.
Then when someone writes an article about building a boat, and links some of the articles in the downloads or files area they would normally copy and paste url, as opposed to putting in some semantic representation, which can be followed, transformed and interpreted nativity by the system.
The only feature not provided by default is the ability to link nodes together, both from within written items (articles), and arbitrarily.
Hopefully soon I will be able to do a good case study on this, from within one of the community groups I am involved in.