- Correctness. It's still possible to generate bad XML. Especially in
	safe mode, checks on processing instructions, comments and DOCTYPE
	declarations should be rigorously matched against the spec.

- Control over presentation. How much is too much? Entities vs. CDATA,
	placement of namespace declarations, whitespace. How much control
	should the user be given?

- Performance. Reducing the use of closures may speed things up, and
	benchmarking would show whether or not it's worth it.
