In fact, recent benchmark testing shows that Server 2008 runs circles around Vista (up to 17 percent faster) at a variety of business productivity and client/server computing tasks.
I made the switch myself about two weeks ago. What I found was an OS that boots quicker and feels more responsive than Windows Vista. All of my applications load faster under Server 2008, while certain classes of application - specifically, managed code - are finally tolerable. For example, Event Viewer no longer runs like a slug in a molasses bath.
1. My "Workstation" 2008 (x64 edition) installation has a lower initial memory footprint after booting. This is true even after I manually hacked the Registry to get SuperFetch working. I also set the Windows Search/Indexing service to start automatically so that my Outlook email is searchable.
2. Disk I/O, in particular, seems smoother. Heavy paging operations, when they do occur, have less of an impact on foreground applications. And, of course, the UI just feels snappier.
3. Some things are moved around a bit. For example, there's no System Restore mechanism. You have to manually enable Shadow Copies for each disk volume.
One benefit of running Windows Server 2008 is that you get to use the wonderful new Server Manager utility. Server Manager is like a central control panel for all the important features and roles that the OS supports. Having all these options in one place saves time and lets you avoid all those extra clicks that the "improved" Vista interface introduced.
Another advantage to running Server 2008 is that you can install locally virtually any Microsoft server application or infrastructure service (Active Directory, Exchange Server).
As a developer, having these services and resources local is a real time-saver. There's nothing more annoying than logging into a remote server or booting into a VM to test an application, only to discover that some minor source code typo is sending you right back to the drawing board. Running Windows Server 2008 as my workstation OS lets me avoid all that and test my code directly from the IDE.
Of course, not everything is perfect in the land of "Workstation" 2008. For starters, there's no Media Center. Media Player is installed, but there are few codecs, and of course you can't play a DVD until you scrounge up an MPEG2 decoder. Windows Sidebar, Fax and Scan, the Games folder - all are MIA under Server 2008.
Ditto Vista's infamous Experience Index, which is perhaps a good move - no point in making Vista look even worse by plastering Server 2008's superior score on the System Properties dialog.