TCP provides guaranteed ,in order, error free delivery. In order to guarantee reliable delivery , the server should employ acknowledgements for confirmation. In case of a problem , it should provide rollback features. The server should maintain a persistent store of information. For this, take regular backups on hard drive, CD ROM etc.
Security is another key concern.
Even if one node goes down , others should be in a position to provide the service. It should provide for client upgrades without necessitating upgrade of server. The server should support multiple client platforms. The server should also support multiple server platforms - from big to medium to chips. Scalability - The server should support multiple users , more CPU's , faster CPU's , more diskspace, more memory.