Here's a helpful article that documents some common pitfalls to avoid when composing XML documents. Nobody wants to be called an XML Bozo by Tim Bray, the co-editor of the XML specification, right?

I'm a little ambivalent about XML, largely due to what John Lam calls "The Angle Bracket Tax". I think XSLT is utterly insane for anything except the most trivial of tasks, but I do like XPath-- it's sort of like SQL with automatic, joinless parent-child relationships.

But XML is generally the least of all available evils, and if you're going to use it, you might as well follow the rules.