It's very complicated. Leading to severe security problems, which means you basically should avoid any untrusted client supplied xml, if you can at all avoid it. It is a step backward from s-expressions, being harder to parse for both machines and humans whilst being less expressive. Whitespace handling is comically bad. It doesn't have a working comment syntax. The list goes on. The only excusably bad thing about xml are namespaces. The design is not a success, but it springs from a list of desiderata which look quite sensible on first sight.