Any software that runs on XP theoratically should run on 2003 fine.
The only exception is Kaspersky personal stuff and a few installation program that specificly checks for XP.
The compatibility stuff in Properties doesn't always work that well.
I’m sorry I can not give more examples. Recently, I needed to work with a program that is something like CAD; it help me draw and asses the necessary material in construction. It’s not too hard to use, but I couldn’t run it on 2003. It just popped a stupid message (I forgot what it said). When tried in a virtual machine with XP and later on a desktop with XP, it worked perfectly.
interesting thought. it would be nice to do something so that a program would recognize win 2003 as winxp or windows 2000 as winxp. most of the programs would work just fine. from what i know win2003 is far stable than winxp. m$ is known to forcefully disable suport for older software. for example office 2003 wants at least win2000.
i beleive that disbling theme service is a good decision. u know that a server must be fast, stable and secure. looking nice is not a priority. i would disable theme service too if i'd use win2003 server as real server and not as a desktop. i know people running linux just in text mode for performance reasons.