Among the good articles written on XML:
Of course, XLL, MathML, and CML interest me. I'm still deciding about OpenTag. RDF is behind everything savvy W3Cers do, as near as I can tell.
Client-side XML mostly makes me yawn. One of the first responsibilities one has in reading much conventional wisdom about XML is to discard its emphasis on client-side processing and rendering.
[Explain inferiority of DHTML.]
[Explain accessibility of XML through such languages as Java, Perl, Python, REBOL, Rexx, Tcl, ...]
[Explain some of the W3C controveries around XML--why XSL is going wrong, for example.]