I was just reading through the XQuery 1.0 language (I'm reading, Priscilla Walmsley's excellent book on XQuery), and found an interesting point specified in XQuery 1.0.
The XQuery 1.0 spec says (Ref):
[Definition: The following axes are designated as optional axes: ancestor, ancestor-or-self, following, following-sibling, preceding, and preceding-sibling.]
[Definition: A conforming XQuery implementation that supports the Full Axis Feature MUST support all the optional axes.]
Giving a little thought about this, I felt that making certain XPath axes (and many of the useful ones, as specified above) as optional in XQuery language is most likely not correct, and would certainly trouble users, who want to uses these XPath axes out-of-the-box from an XQuery engine.
Interestingly, the latest working draft of the XQuery language (the, 1.1 version) has fixed this design mistake (I think, this was a design mistake!), and doesn't specify such a constraint in XQuery language.