Unlike Windows Client, Windows Server isn't particularly my forte. So, I'm kind of relying on others who have better understanding of what's being changed. While I might not be particularly versed in these matters, I do get the three most important themes, though.
First, Windows Server Core has been given more prominence. Up until now, Server Core has been problematic because most Windows server software depended on the graphical user interface, and in order to switch between Server Core and Server GUI, you needed to perform a re-install. This is no longer the case - the graphical user interface is now merely an isntallable package. Yet another benefit of the componentisation of Windows.
There are now three different modes to run Windows Server in: the existing Server Core (no GUI at all) and server-with-everything-in-place, and a new mode that sits somewhere in between, which doesn't actually launch the entire graphical stack, but only the new Metro Server Manager and MMC.
Second, the new Server Manager itself is different in that it is focused on PowerShell. As Ars' Peter Bright explains: "[Server 8] centers around PowerShell and Server Manager, the new Metro-style management console. Server Manager provides a convenient GUI, but behind the scenes, PowerShell commands are constructed and executed. The commands can also be copied, edited, and executed directly in PowerShell," he details, "This should sound familiar to many Windows administrators, as Exchange already uses this style of management, with the GUI being a mere layer over PowerShell."
The third and final big change is in the field of virtualisation - Server 8 comes with Hyper-V 3, which contains many improvements I honestly don't always understand in full detail. I can copy/paste whatever Ars has to say on this, but it's better to just head on over there and read it all in full detail. More information can also be bound on the Windows Server 8 site itself.
The developer preview for Windows Server 8 is available just like the client version, so if you want to play with it feel free to do so - you do have to be an MSDN subscriber, though.