- 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.