Meanwhile, the datatype side is ugly but ubiquitous

Odd collection of primitive types There are nineteen built-in primitive types, a lot of which are special-purpose and there is no way (except by new Recommendations) to create new primitive types.
Derivations are odd Who derives integer from decimal? Really? Apart from the primitive types, all the built-in types are derived from either string or integer.
Limited flexibility WXS data types provide no support at all for things like localized date and number formats.
Regular expressions common escape hatch The pattern facet is regularly used to create new types that WXS doesn't know about - except that it only affects the allowed values, and can't be used to create new facets or other type-specific information like the interpretation of the value space.
Not easily mapped While there are lots of types, they don't map that well to existing systems. To put it politely, WXS datatypes are a strange mix of database, programming, and XML 1.0 types.