Tiles framework has got a new face lift; to include tiles web.xml has to be updated with a new init-param called chainConfig and will have to point the param value to chain-config.xml. The new struts config DTD is located at
http://struts.apache.org/dtds/struts-config_1_3.dtd. The moduleAware param of tiles has become more sensitive. So until you are sure to use it don't even bother changing the default value. It really is a pain to debug what is happening. One good thing with tiles now is the global-forwards.
The JSP's will have to include new DTD's for the struts standard TLD's
Error Handling: Atlast the error handling in struts 1.3 got streamlined well. They have completely stopped the support of ActionErrors. Happily use only the ActionMessages, the mix match of ActionError and ActionMessage was such a pain in every team. Standardizing that is probably one of the best possible thing.
One thing I was not sure with this upgrade is to understand the power of Chain of responsibility pattern. I intend to spend some more time on it to get some notes here.