I have been working with XML since it was a glimmer in the eye of Jon Bosak. In fact, before XML was conceived, there was SGML; this evolution of SGML represented a streamlining for the web, but at its core there was not much functional difference; in fact the new invention was defined as a mere SGML subset. The key concept of semantic markup is central to the core value of SGML as well as its "streamlined for mass consumption" child.
The two main perspectives I have seen are Document-centric and Data-centric. SGML initially appeared in support of document-centric work: managing all the technical documents or contracts of IBM or Boeing, for example. Charles Goldfarb has maintained that "SGML literally makes the infrastructure of modern society possible" and I think he's right - hmm, should we blame him for the lengths to which humans have gone to destroy the earth?