The question about what features are supported by SQL Server 2005 running on Windows Server 2003 comes up quite often. So below are some of the features that are depends on the OS and brief description about them.

      
1. Password policy/expiration check for SQL logins - CREATE LOGIN is a new DDL for creating SQL logins. It has two options called CHECK_POLICY and CHECK_EXPIRATION that can be used to enforce Windows password policies for SQL logins. This leverages the functionality provided by NetValidatePasswordPolicy() API

2. Hot add memory - This is a new feature in Windows Server 2003 that can help SQL Server 2005 indirectly
3. Dynamic AWE - SQK Server 2005 running on Windows Server 2003 uses dynamic AWE memory allocation. This works similar to regular memory management

4. 64-bit versions of SQL Server 2005 runs only on Windows Server 2003 64-bit (IA and X64) - Native 64-bit editions of SQL Server 2005 runs only on corresponding Windows Server 2003 64-bit editions

5. SOAP support - Native XML web services in SQL Server 2005 relies on HTTP API support which is provided by Windows Server 2003 and Windows XP Service Pack 2

6. Fast file initialization - Creating a new database of any size is almost instantaneous in SQL Server 2005 running on Windows Server 2003/XP. This requires the service account to have the SE_MANAGE_VOLUME_NAME permissions. With fast file initialization, disk content is overwritten when new allocation happens

7. SQL Writer service features that work with Volume Shadow Copy Services - Differential backup, differential restore, restore with move, database rename, copy only backup and auto-recovered Snapshots are supported only on Windows Server 2003 with Service Pack 1

 In addition to these features that can benefit SQL Server installations on Windows Server 2003 platform, there are other inherent advantages. Windows Server 2003 is more scalaeable and secure than Windows Server 2000. These are just some of the things to keep in mind when choosing a platform for running SQL Server 2005. The link below contains some benefits on using Windows Server 2003 with SQL Server 2005:

http://www.microsoft.com/sql/techinfo/planning/winsvr2003benefits.mspx