An enterprise application is an application which is suitable for use in a wide variety of environments seen in companies. (for example, as a point of difference, an enterprise application might support a wider variety of platforms or data, or at greater scale than a non-enterprise application)
What about collection of different applications which is inter-dependent with each other. Each application must be communicable and extensible with less pain. That's what in my mind for enterprise right now..
Multiple standards exist for just about anything, and most implementations of something are broken in one way or another. I've seen obvious and subtle breakage in things that ought to be simple: * timestamps (not unambigious, not including timezone data) * mac addresses (within SNMP, in a variety of formats in different places on the same kit) * integration with LDAP (group membership only looking at primary membership) * etc.
We live in hope. Postel's Law applies.