- ORM enhancements with support for more relationship types, easy transactions and criteria building, constraints to SQL schema mappings, and upgrade to Hibernate 3.2
- All-new non-invasive plugin system for writing reusable plugins for Grails applications (http://grails.org/The+Plug-in+Developers+Guide)
- Greater Spring integration thanks to a new syntax for scripting Spring (http://grails.org/Spring+Bean+Builder) and an upgrade to Spring 2.0
- A new Groovy-based build system based on Gant for easy command line scripting (http://grails.org/Command+Line+Scripting)
- Grails now ships with Groovy 1.0!
Read the full announcement at http://www.nabble.com/-groovy-user--Grails-0.4-Released!-tf3150030.html. Change log at http://jira.codehaus.org/browse/GRAILS?report=com.atlassian.jira.plugin.system.project:changelog-panel.