My five-point test requires a new release to bring at least five significant improvements to my environment, or it's not worth upgrading. Each improvement has to change my life in a significant way, either by dramatically shortening the time to do common tasks or by allowing me to do something I couldn't do before. For my environment, which is a large data warehouse, these five Katmai features easily pass the test: Change Data Capture, Lookup Cache, Data Compression, PowerShell integration, and Policy-Based Management. And I could easily keep counting.
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Change Data Capture and Lookup Cache will be popular with DBAs who want to speed up ETL (Extract, Transformation, and Load) processes, as will the pipeline improvements in Integration Services, enabling it to push data much faster. Data Compression, PowerShell integration, and Policy-Based Management, not to mention Server Groups, should make a big splash in almost any environment. Just keep in mind that the benefits you get from Data Compression (which works at the page level, replacing repeating data with lookup pointers) will depend on the nature of the data in your table and how it's ordered.
Backup Compression is also new in SQL Server 2008. Although the compression itself works well, the feature has too many limitations (including supporting only Enterprise Edition) to be effective in an enterprise setting. Another compression feature, called Sparse Columns, allows you to store nulls without taking up physical space. Sparse Columns will come in handy for large tables containing many null values. It's just too bad you can't use Sparse Columns and Data Compression on the same table.
Policy-Based Management allows you to define almost any configuration or administration policy you might think of for any number of servers, and be alerted whenever those policies are violated. Although spanking new in Katmai, Policy-Based Management already seems mature.