Well, Server 2008 has Hyper-V, albeit an older version. Assuming you used it on 64-bit hardware with lots of memory, then you can virtualize many of your old, underutilized WS2003 servers onto less hardware.
Server 2003 is a 32-bit OS while Server 2008 is 64-bit. That means WS2003 is limited to 4GB while Ws2008 can handle a terabyte of memory (2012 handles 16TB). So that's a lot more room for databases and web servers, the ability to handle many more users, etc.