- Not including self-closing tags, there should only be one close tag: </>
- Elements are for data. Attributes are evil
- XPath indexing should be 0-based
- Documents without a schema should not make your tools panic or complain
- An xml document shouldn't have to waste it's time telling you it's an xml document in xml
I maintain that one of the reasons JSON got so popular so quickly is because it does all of the above. The problem with JSON is that you lose the benefits of having a schema to validate against.