is web-inf/lib should contain only application specific jar's or deployment environment specific jar's also as per J2EE standards.
Note : war file is not making use of any deployment environment specific functionality (for instance weblogic or websphere or tomat etc ...)
The WEB-INF/lib folder should contain your application specific jar files. Jar files that are global to the server will be stored in the server's classpath.
Application specific jars and if you have multiple applications deployed on the same server/JVM then any jars whose classes (class instances) you don't want to share. For more details about my second point: my earlier question about classloaders