Henrik Walther wrote a nice article on Exchange 2010 for the December edition of TechNet Magazine:
The recently released Microsoft Exchange 2010 Release to Manufacturing (RTM) version, like its predecessors, includes a wealth of new features and improvements over existing ones. As a matter of fact, with this release, Exchange now consists of approximately 21 million lines of code.
Exchange developers had five main goals in mind for Exchange 2010: to help organizations achieve new levels of reliability, better performance, simplified administration, improved protection of communications and greater business mobility for users. In short, and with the global economic crisis in mind, they aimed to create a more flexible and optimized product that would lower the costs of running an Exchange 2010 infrastructure.
Since April 2008, I have spent a good deal of time testing Exchange 2010 beta and release candidate builds in my labs, as well as in two enterprise customer environments. In this article, I will take you on a tour through the most exciting changes and improvements in this latest and—without doubt—greatest Exchange version we have seen so far.