Recently I have completed migration of my applications from 4.0.3SP1 to 5.1.0.GA.
Below are the Challenges I have faced
1. My old application is usi.ng 2.2 web.xml dtd. So I upgraded web.xml dtd version to 2.3.
2. In 5.1.0.GA dtd validation is strict. So web.xml entries need to be in order according to the dtd.
3. I have upgraded my applications to hibernate 3.0.5 to 3.3.3.GA. practically there is no change in the application. And also I have not bundled the hibernate jar in my war file.
4. I am using facelets 1.1.9 It is not working in JBOSS 5.1.0. I have upgraded to facelets1.1.15.b
5. Added an entry in web.xml which specifies JSF jars bundled in the war file.