This list of enhancements is making my mouth water: http://java.sun.com/j2se/1.5.0/docs/guide/jws/enhancements-1.5.0.html
So many awesome improvements, I especially like:
Import Facility: Java Web Start now contains an import facility. This can be used for CD installs, where code is initially loaded from one location and then updated from another. It can also be used to pre-install applications and libraries in either the user or system cache without running the applications.
VM arguments: A JNLP file can now request arbitrary arguments to the virtual machine. Previous JNLP applications could only use the initial-heap-size and max-heap-size arguments to the j2se element; now a java-vm-args argument has been added. Java Web Start will honor the requests for any normal java and -X arguments that it considers "safe". A list of all such arguments will be included with each release so that the JNLP Spec does not need to change.
The second reduces the number of things you can't do in JWS but could with the plain java command, and the first significently improves the ability to deploy JWS applications by other means and use JWS to keep the clients updated.
Another nice feature I like is this one:
JNLP Cache Viewer: The JNLP Cache Viewer replaces the functionality of the Application Manager's "downloaded Applications" view. Now all applications, applets, libraries and installers are shown. Available applications can be launched online or offline, removed, or restored from the web. All JNLP files are now kept in the cache (even those without href parameters).
Significently the "offline" version which is useful in a game situation where you may want to hold off updating until the servers have first been updated (obviously for games with where the servers are controlled by individuals).
Nice going sun!