Although these are the roles which are built into the Server production, you should not overlook the other roles available from other Microsoft Server platform products - for example, Exchange Server 2007, SQL Server 2008, the various data production services, and not forgetting the various management tools in the System Center range.
It would be naive to think that organisations will be deploying pure Server 2008 without at least some of the other services in place. From experience, the System Center packages should almost be considered mandatory if you are to keep a solid management eye on a larger (or even smaller) network installation. From that point of view, the roles in Server 2008 should be seen as only part of the overall picture.
For the first time, there is a wholly lights-out version of Server, called Server Core. This lets you run with essentially only a command line interface, and minimises all the GUI functionality. After some 15 years of solid GUI operation, this comes as something of a shock to a seasoned Windows Server administrator, but the option is welcome and it will certainly have much traction for use with rack mounted servers, for example.
For any sort of lights-out installation, there is no need for the GUI to be running. All management functions can be run over the network from a GUI-supporting workstation, of course. Not all roles will work on server core - Microsoft's list includes the Web Services (IIS), HyperV, AD Domain Services, AD Lightweight Directory Services, DHCP, DNS, Print and File. Clearly many of these are a good option for running within a hypervisor virtual machine session too, and hence the reduction of memory requirement of the Server Core installation means more VM's can be fitted into a given amount of RAM on a hosting server.
For web hosting, both of static sites and complex application services, IIS7 has been considerably enhanced and improved. In a move which will bring relief to those managing large IIS farms, the configuration information is now stored in a simple Web.config file, which can be copied using file system tools between any number of servers. This also allows for rapid change management and disaster recovery too.
For application serving, much work has been done to separate out the various sessions and configurations between various running processes. As you would expect there is much here which is reliant on the Windows WinFX .NET Framework version 3.0. To help with the installation and configuration of this, the new role of Application Server is designed for those administrators who want to deploy line of business applications using the .NET 3.0 framework.