RELAX NG has different foundations

Patterns Nearly everthing in RELAX NG is a pattern, or set of patterns. The patterns are descriptions of structures that can be found in XML documents, not abstracted types.
Hedge Automata While the name sounds prickly, and the math is difficult if you don't speak math, having a clean mathematical foundation makes it much easier to avoid some of the performance and structural issues that creep into more ad hoc systems.
Related to Regular Expressions Hedge automata come from the same field of set theory that created regular expressions. In a lot of ways, RELAX NG schemas are like regular expressions for document structures rather than the character structures regular expressions describe.
Never touch the document The core of RELAX NG only describes the structure and contents of XML documents. Unlike W3C XML Schema, there is no effort to supply default values or annotate documents with additional type information, though RELAX NG does support a separate DTD-compatibility layer to provide the functionality DTD users expect. (Neither RELAX NG nor WXS provide a replacement for DTDs' entity declaration capabilities.)