Sean left a comment referring to a post by James Clark on TEDI, a new kind of schema language for XML. It's a very interesting post. I like how it elevates the issue to one of defining a common data meta-model, with mappings to the plethora of formats which are all basically trying to do the same thing (XML, JSON, YAML, etc).
However, I think I noticed something missing in the TEDI concept. This is the ability to define complex types which are composed of combinations of basic data structures. XML Schema of course does have this feature, and it's responsible for a significant part of the perceived complexity. But a schema language which lacks this ability is limited in its expressiveness. A sterling example is GML Geometry. Geometric types are complex enough that they have to be defined in terms of simpler XML data structures. One of the goals of GML is to provide a meta-schema which defines the concept and structure of expressing geometric datatypes in XML. To accomplish this, the concept of a Geometry datatype needs to be expressible at the schema level.
Of course, for any given application domain you can dodge this issue by simply relying on convention. A datatype with more than trivial semantics is going to have a custom implementation in any given system, so a custom mapping from XML will be needed in any case. But that's not really the point - surely at this stage of computer science we can do better than arbitrary convention?
I haven't yet seen an explanation of whether RELAX-NG provides this level of expressiveness. I don't see it in JSON or YAML. But it seems like this capability is essential for any schema language which is going to take us beyond simply collections of a small set of datatypes.