There are docs on www.springframework.org for Spring 3.0 called What's new. That will tell you everything.
Keys to Spring 2.5 to 3.0
3.0 requires Java 5. 2.5 can use Java 1.4
Spring Expresssion Language
More Annotations, covers everything that you could do in xml
Adding Java Config module to Core Spring
Spring RESTful Web Services are new
for MVC, more Annotations, Spring RESTful WebServices, which includes HttpMessageConverters, ContentNegotiatingViewResolver and MVC Namespace
But everything that you could do in 2.5 you can also do in 3.0.
For everything else, you will need to check the documentation on Spring's website.