Originally posted by ss_java: what is the criteria for a server to act as an application server. why exactly cant we use tomcat
Ehm, because its not an Application Server, so its the wrong tool for the job in the same way you wouldn't try to use, say, Macromedia Flash to persist relational data for example.
Sun provides the J2EE spec (search for it on their site) which defines what services a platform must provide to be considered an Application Server. Further to that they also certify servers software as J2EE compliant applciation servers, if the vendor wishes to pay for certification.