Policy-Based Management and PowerShell integration are the biggies, but Management Studio has added some other nice features, such as Server Groups, which allows you to run queries against multiple servers at once. However, I'm not fond of the new Activity Monitor. It may be a step in the right direction, but it just isn't useful in its current state.
Another new feature that isn't quite there yet is the ballyhooed Resource Governor. The Resource Governor lets you define limits on CPU and memory usage for certain workloads. This is good enough to prevent some traffic jams -- like query processes consuming too many resources on an OLTP server, for example -- but it's a far cry from what's needed to define and isolate rogue queries.
In short, SQL Server 2008 is a solid release packed with significant improvements. Not all of the new features are fully fleshed out, but some are surprisingly mature and many are extraordinarily useful. A few will allow DBAs to manage multiple servers as easily as one, and perform certain tasks and operations in a fraction of the time they take today. Read on for more detail on SQL Server Management Studio and management improvements, Resource Governor, Change Data Capture and nonlogged inserts, Data Compression, Backup Compression, index improvements, SQL Server Reporting Services, and SQL Server Integration Services.
If there's any one new feature in SQL Server 2008 that will change the way DBAs manage their environments, it's probably Server Groups. Server groups let you run any query you like against an entire group of servers simultaneously. So instead of having to cycle through all of your servers to check job status, deploy stored procedures, and so on, you can manage or push code to any number of servers with a single query. On top of that, Server Groups can be further extended and enhanced to do some great things. It does take a paradigm shift, however. DBAs will have to start thinking in terms of groups instead of single servers.
SSMS (SQL Server Management Studio) also has much better object-level stats than ever before. This is a perfectly executed feature, and I love it. In the object details pane, you can now click on an object folder and get info for all objects in that folder. For example, click the Databases folder to immediately see the key info on all the databases on the server, including size, space used, owner, index space used, and many more. This is an incredibly useful list because you can sort it and reorder the columns and your customizations persist. You can get detailed info on all of your tables too. In this new detail view, you can see row count, data and index usage, schema, and so on. Not only is it very easy to view object-level stats, but this level of info is available for all folders, not just databases and tables. This is one feature that I wouldn't change in any way.
One of the most important features included in this release is PowerShell. PowerShell is going to change how DBAs manage their environment, because it allows you to do complicated things so much easier than in Visual Basic or T-SQL.