> Why does // expand to descendant-or-self::node() rather than 
> to descendant::x where x is the next item in the path?

I've always imagined it was primarily to make //@x work; though I'm not at
all convinced it was a smart decision. Certainly in practice it's very rare
to see any axis other than "child" after "//" (for example //.., or
//following-sibling::x is never seen in practice and never required; and
//@x is something one could live without).

I think making "//" expand to "/descendant::" might well have been a better
design.

Michael Kay
http://www.saxonica.com/