Goals of Web Ontology Language
Supports shared ontologies: e.g., Using URI as identifier in RDFS.
Supports ontology evolution.
Supports ontology interoperability.
Facilitates inconsistency detection.
Makes balance of expressivity and scalability: RDF/S is very scalable but is not very expressive, e.g., it cannot specify that Person and Car classes are disjoint, or that a String-quartet has exactly four musicians as members.
Ease of use, XML syntax and Internationalization.