Now, in general, there will be no
problem implementors who have a homogeneous target audience and a very
narrow scope of their service, but life is not as easy as that. Let us
take a real-world example...
Metadata is used for describing
any kind of data in order to make those data easier to find. Some metadata
are absolutely essential, and a web browser just cannot do without them,
for instance, the browser need to know the physical format of a retrieved
resource or it cannot be rendered by the browser or select the appropriate
plug-in or helper application. If such metadata are incorrect, it will
generate an error in the browser, and in a sense, metadata is the data
needed to make information out of what is otherwise just an disorganised
digital blob of zeros and ones.
Apart from the very simplest
types of metadata (like mime type and content size), one of the main problems
of metadata is that there is a large number of standards available. Each
of those standards have their own application area; that is, each of them
are good for describing certain kinds objects. So, for instance, MAchine-Readable
Cataloging (MARC)  has been used for decades in the description of
all kinds of media in the library communities. It is also used by archivists
for describing archival collection, but this community is now more and
more turning to Encoded Archival Description (EAD) . I could go on
and on and give acronyms for metadata element sets, both established standards
and new projects. In the education sector, we see the IMS Project ,
DC-Education and the UK Metadata for Education Group (MEG) . There
are more of these, both initiatives geared towards education or other
sectors of society or industry.
Now, in general, there will
be no problem implementors who have a homogeneous target audience and
a very narrow scope of their service, but life is not as easy as that.
Let us take a real-world example. Go to the US Department of Education
and search for the acronym GILS . All federal governmental agencies
in the US are by law required to provide metadata for reports in the Governmental
Information Locator System (GILS) . You will find a few such records
on their site, and also tens of thousands of them in a search service
hosted by ERIC. It is an obvious advantage that governmental information
of all kind is made available through a single metadata standard. But
the same information, one could argue, should be available on public libraries,
where they have to be search-able through MARC in a library OPAC and perhaps
through IMS in educational databases. You already see the problem, I suppose:
The problem of interoperability. The same data may need metadata for different
purposes, for different situations or target audiences.
One of the first initiatives
to address this problem was the Dublin Core Metadata Initiative (DCMI)
. The Dublin Core metadata element set is meant to represent the least
common denominator of the more specialized, and also more complex, element
sets used in specific communities. In addition, the people designing the
DC had in mind the functional requirements for simple resource discovery
on the Internet. The effort has been successful. Just about any detailed
description should be able to 'down-grade' into DC, regardless of the
source element set used to create it in the first place.
But down-grading has its price.
Information is lost, and what has been down-graded can nerver be up-graded.
And it has to be done with care, or the result might be erroneous or misleading.
New ideas are now emerging where a more atomic approach is taken in the
task of creating a metadata element set.
New views on metadata are now
possible because of the development of new technologies such as XML 
and RDF . One such attempt is the Harmony project  which attempts
at building a general framework for metadata vocabularies . In the
future, metadata elements will not be thought of as living in specific
element sets. Rather, implementors will be able to build their applications
using elements from different sources, called namespaces . These new
kinds elements, which are reusable entities made available in different
namespaces designed for different needs, will if chosen with care, fit
together just like LEGO pieces. The Harmony project is now building a
metadata element thesaurus for testing this idea. Such a thesaurus will
contain information about the relations between metadata word, such that
Director, Author and Illustrator all have the same broader term, Creator.
Subject has narrower terms Classification and Keywords. So in the future
we might see one single service, which using a modular approach to resource
- Interoperability with library
OPACS through MARC and library classification systems.
- Having simple access interoperable
with Internet search services through Dublin Core.
- Use geospatial metadata
for geographical retrieval of geographical teaching resources.
- Allow searching based en
educational metadata using IMS 
We are not there yet. And,
when I think of it, noone has volunteered to create the metadata.