The most important thing is that thanks to modularized architecture it's easy to replace any part of a system. You can change database, or MTA part easily. Software can be configured without the need of compilation.
Main features are:
PostgreSQL support for authorization and logging
JDBC support for authorization and logging
Qmail integration
CORBA based communication, which means you can use daemons from any language having IDL mapping
can be used easily from: C, C++, PHP, Java, Smalltalk, python
distributed hosting
all processes can run without root
written using C++, STL and boost, some parts in Java
WWW interface for users