Computer programmers want data to be as structured as possible. If you don't give users a lot of room to do unpredictable things, it's easier to write software that does cool things. Users on the other hand, want (or think that they want) total control over data and the ability to do whatever they want.
The problem is they don't. Most digital collateral, even the content stored in unstructured formats, is pretty structured. While people may want freedom, they don't use it, and in many cases users go through a lot of effort to recreate structure within unstructured forms.
Structured data are data that is stored and represented in a tabular form or as some sort of hierarchical tree that is easily parsed by computers. By contrast, unstructured data, are things like files that have data and where all of the content is organized manually in the file and written to durable storage manually.
The astute among you will recognize that there's an intermediate category, where largely unstructured data is stored in a database. This happens a lot in content management systems, in mobile device applications, and in a lot of note taking and project management applications. There's also a parallel semi-structured form, where people organize their writing, notes, content in a regular and structured manner even though the tools they're using don't require it. They'd probably argue that this was "best practice," rather than "semi-structured" data, but it probably counts.
The less structured content or data is the less computer programs are able to do with the data, and the more people have to work to make the data useful for them. So while we as users want freedom, that freedom doesn't get us very far and we don't really use it even when we have it. Relatedly, I think we could read the crux of the technological shift in Web 2.0 as a move toward more structured forms, and the "mash up" as the celebration of a new "structured data."
The lines around "semi-structured" data are fuzzy. The trick is probably to figure out how to give people just enough freedom so that they don't feel encumbered by the requirements of the form, but so much freedom that the software developers are unable to do really smart things behind the scene. That's going to be difficult to figure out how to implement, and I think the general theme of this progress is "people can handle and developers should err on the side of stricture."
Software like org-mode and twiki are attempts to leverage structure within unstructured forms, and although the buzz around enterprise content management (ECM) has started to die down, there is a huge collection of software that attempts to impose some sort of order on the chaos of unstructured documents and information. ECM falls short probably because it's not structured enough: it mandates a small amount of structure (categories, some meta-data, perhaps validation and workflow,) which doesn't provide significant benefit relative to the amount of time it takes to add content to these repositories.
There will be more applications that bridge the structure boundary, and begin to allow users to work with more structured data in a productive sort of way.
On a potentially orthogonal note, I'm working on cooking up a proposal for a LaTeX-based build system for non-technical document production that might demonstrate--at least hypothetically--how much structure can help people do awesome things with technology. I'm calling it "A LaTeX Build System."
I'd love to hear what you think, either about this "structure question," or about the LaTeX build system!