Don't know if I'd say a "heap" but there are some. Noteably fine grained
password policy and a few others. The only got ya, is there is no going
backwards on domain or forest functional levels. If it is already been
raised, then you cannot use 2003 DC's.
You can find out more details here;
http://technet.microsoft.com/en-us/library/cc754918(WS.10).aspx
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/kj