A standing complaint about Windows Server is its resource footprint. Those in IT just take as rote that it requires lots of memory, lots of CPU, and lots of disk to put any substantial services on the air with Windows Server 2003. I think it's safe to say that the typical x86 rack server's characteristics reflect the requirements of Windows Server. Microsoft's big OS has always been designed under the presumption that it will have a full physical server to itself.
In Windows Server 2008, Microsoft delivers a 64-bit server OS with a smaller minimum resource footprint than Windows Vista. It varies by edition; Windows Server 2008 Datacenter doesn't focus so much on shedding the pounds, but it, too, picks up the speed benefits from the slimmer Server Core, which was created to be a practically weightless virtualized guest OS. IT shops are likely to use Windows Server 2008 the same way they use Windows Server 2003 now, only now they can run lots of independent virtual Windows Servers that scale in features and footprint across a broad range of options.
Windows Server 2008 remains a component of the Windows Server System, so Microsoft has not instituted a free lunch program. Functions like e-mail and collaboration, database, and robust edge services are add-ons that most deployments will require. But these can be placed at the host level, with virtualized guests distributing applications and services that utilize Windows Server components. In other words, one license of Exchange Server or SQL Server will stretch further than ever before.
How low it can go
I spent most of my time testing Windows Server 2008 Enterprise on an eight-core, two-socket AMD Barcelona reference server. When you align the features of the Barcelona architecture with Windows Server 2008's capabilities, you come away with the impression that AMD designed its CPU with Windows Server 2008 in mind. Having talked with Barcelona's architects, I'll bend nondisclosure just enough to say that to call Barcelona a Windows Server 2008 hardware architecture is not far-fetched.
Windows Server 2008 is built for virtualization. All SKUs up to Datacenter are tooled for what you might call "buffet" scalability. You can choose, with finer granularity than is possible under Windows Server 2003, the server features you want to run, where you want to run them, and what portion of total resources are dedicated to them. For example, Internet Information Services (IIS) 7.0 has split Web application services functionality into some 40 independently loadable plug-ins. It is similar in concept to Apache's modular approach, but IIS's approach is safer, more transparent, and much easier to manage.
This is a nice fit for server roles, a feature introduced in Windows Server 2003 that provides simple on/off switches and wizards that bring up and shut down groups of services according to need. Windows Server 2008 continues Windows Server's tradition of server roles, but adds finer-grained, modular control over individual features. You can still do a blunderbuss deployment in which a Windows Server host or guest role is "all," but it is well worth IT managers' and administrators' time to learn to match server roles, and modular services within those roles, to user and application requirements. Do that, and you'll have servers that will make physical-to-virtual transitions and virtual machine relocation uncommonly easy.
One road you won't need to take to slenderize Windows Server 2008 is to run it as a 32-bit (x86) OS instead of 64-bit (x64). You've heard hype that the overhead of going to 64 bit, especially for virtual guests, is substantial enough to blow x64 off unless you know you need access to a 64-bit virtual address space (as if that knowledge were easy to come by). Dismiss this as noise. The 32-bit server OS is the HD DVD of IT, even for virtual guests. It's time to step into the future.
To put a fine point on the virtues of Windows Server 2008's trimmer physique, consider that I ran the x64 Windows Server 2008 Standard on an Apple MacBook Pro, running as a 64-bit virtual guest under VMware Fusion software virtualization for OS X. Of MacBook Pro's 2GB of RAM, I reserved 512MB for Windows Server 2008. I made just one allowance for Windows Server 2008: I installed it on an off-board 18GB FireWire-powered hard drive. To be honest, that was for me. I wanted a blinky light that showed me how hard Windows Server 2008 was hitting the drive.