For a start SiXDML provides a hybrid syntax that combines the familiar SQL format with XPath and other query languages. Secondly the driver provides transaction support, which is a must in any serious environment these days.
This allows you to quickly leverage the very powerful hierarchical nature of XML databases through their ability to query objects at any depth in the document collection. Which frees the data model from the rigity and prescriptive nature of RDBMS.
There are some features worthy of any killer app too. Let your imagination wander...
SELECT //myobj FROM COLLECTION mydata AND TRANSFORM WITH XSLT IN http://www.mine.com/transformer.xsl...nice