There isn't any real consensus on what Web 3.0 is as of the spring of 2012.

Suggested key features are:

• Data/Semantic Web - Inclusion of semantic content in web pages to promote common data formats.
  The semantic content will be in the form of metadata in web pages.
  Some standards for this include:
  Resource Description Framework (RDF) - Similar to classic conceptual modeling approaches such as entity-relationship or class diagrams
  and Web Ontology Language (OWL) - Allows applications to process the content of information instead of just presenting information to humans.
  

  See Tim, Lucy, and The Semantic Web - Web 3.0 | PCMag.com

• Personalization - Content customized for users based on their implicit behaviour and preferences.
  Keywords will be searched based on your culture, region, and jargon.
• Metaverse - Convergence of the virtual and physical world. e.g. Geosocial (Foursquare, etc.)
• The web as a personal assistant. At HowStuffWorks they suggest you will be able to ask "I want to see a funny movie and then eat at a good Mexican restaurant. What are my options?"