I've always thought that XML has been oversold — XML is very useful, but only in an abstract sort of way. It's just one tool that software developers can use.
I particularly liked this bit of a Dr. Dobb's Journal interview of James Clark in the July 2001 issue:
DDJ: What's the next step for XML?
JC: That's a difficult question. I think XML has become so widespread, it's like asking me, "What's the next application for ASCII text? What's the next application for line-delimited files?" XML is becoming so common, it's not interesting [for its own sake] anymore.