There are a number of things that aren't trivially do-able with the tools in Windows Server 2003.
- You can't easily set up an automated backup tool like that in Windows Home Server.
- You don't have a Remote Access web site that will let you browse/download/upload files.
- You don't have an easy way of controlling your server remotely.
- You don't have easy access to your home PCs through a secure proxy.
- You don't have an easy to use management interface.
- You don't have an easy way to add functionality created by third parties.
There's lots I'm leaving out. The bottom line, though, is that you will give up all the ease of use features. In exchange you gain greater flexibility and the ability to do things that are effectively forbidden by the WHS architecture.