Encryption

• SQL Server 2005 provided no facility for database encryption. In 2008, Transparent Data Encryption (TDE) enables you to encrypt an entire database, including metadata and user data. Additionally, 2008 permits encrypting backups.
  
  

Enterprise Management

• Organizations may only deploy SQL Server 2005 servers as stand-alone or failover cluster configurations, making management of large numbers of server instances challenging. SQL Server 2008 allows you to designate a Central Management Server, from which Transact-SQL statements and policy-based management (PBM) actions can be deployed against groups of SQL servers.
  
  

Resource Governor

• SQL 2005 did not permit limiting the resources that SQL Server makes available to queries, but SQL Server 2008 includes a resource governor to do so.
  
  

Installation

• SQL Server 2008's installation process for both command-line or graphical user interface (GUI) deployment requires many fewer steps than SQL Server 2005, and includes more intuitive wizards for deploying additional features and failover clusters.
  
  

Auditing

• SQL 2008 enables organizations to audit changes to databases without impacting the performance of the server. In SQL 2005, auditing could only be enabled through the use of third-party products or resource-heavy custom scripts.