I just upgraded my test server to 6.0 and ran into some troubles that
maybe others would appreciate hearing about.
First there was the mysql root password issue as reported at
http://jira.xwiki.org/browse/XE-1396
Xwiki still wouldn't start after the upgrade and
/var/log/tomcat7/localhost.log said "SEVERE: Error configuring
application listener of class org.xwiki ... Unsupported major.minor
version 51.0"
Turns out my server had java6 AND java7 installed and I think tomcat was
picking up 6 which no longer works for Xwiki 6.0. So I opened up
/etc/default/tomcat7 and set the JAVA_HOME var to the java7 folder and
now xwiki works again.
Hope that helps someone out there... :)