Work, Expression, Manifestation, Item
It is often easy to miss something that is obvious when pointed out. When identifying resources it is easy to be a little sloppy and fail to discriminate between 4 concepts that have been called "work", "expression", "manifestation" and "item" in the rather interestingly-acronymed FRBR. This isn't an emoto-tronic friend but Functional Requirements for Bibligraphic Records (may be found at http://www.ifla.org/VII/s13/frbr/frbr.pdf).
A work is the intangible product of intellectual or creative endeavour, whereas an item is a tangible thing that I could own. For example (borrowed from FRBR): J. S. Bachs Six suites for unaccompanied cello is a work, the performances by Janos Starker recorded in 1963 and 1965 are an expression and the recordings released on 33 1/3 rpm sound discs in 1965 by Mercury are a manifestation.
If software systems are built to correctly handle these different cases then quite a number of user benefits can be realised. For example, by allowing the location of available resources that are of the same work when all I know is the identity of a manifestation. The manifestations of an expression could differ in the medium, for example microfilm vs print for conventional publications. It is at this level that some accessibility issues could be dealt with.
For an example of where FRBR has inspired an e-system, take a look at ePrints Application Profile.