Windows Server 2003 brings a number of feature improvements and new capabilities to Active Directory. One of the biggest surprises of Windows 2000's lifecycle was the number of companies that adopted Windows 2000 but continued to run Windows NT-based domains, rather than moving to Active Directory. In Windows Server 2003, Microsoft has attempted to address some of the issues that discouraged companies from adopting Active Directory, while maintaining full backward-compatibility with Windows 2000 Active Directory as well as Windows NT-based domains.
Active Directory's new features fall into five basic categories: tools and utilities, administration, architecture, security, and operations. None of Active Directory's changes or new features are really earth-shattering; instead, they're the type of sensible, gradual improvements you'd expect in a version release of a product.
You can migrate directly from Windows NT domains to Windows Server 2003 Active Directory or from Windows 2000 Active Directory. However, Windows Server 2003 doesn't make Active Directory planning any less important to a migration process. If anything, Active Directory's new features make planning even more important.