• Copy and paste using the Project Navigation
• Refactoring (move to package, etc)
• Code completion (for example if you start a JavaDoc comment, IntelliJ doesn't close it automatically like it does in Java source code) unless the package definition is typed into the file by hand, but since there isn't any Create New support, this is slightly cumbersome.
• Intelli-sense (you cannot complete annotations because they must be fully-qualified and IntellIj doesn't seem to know how to complete fully qualified annotations)
• Create New package-info.java from the context menu
• Really no error highlight at all
• No quick javadoc support for package-info.java files. For example, if you hit ctrl-q on an import statement for the package, it should pull in the JavaDoc that is located in the package-info.java file in the source JAR. Also, if you are in the package-info.java file and working on JavaDoc, you should be able to hit ctrl-q to preview your work.

Also, it might be nice to automatically insert a package-info.java class into each newly created package. This could be an option so folks could turn it on and off.