One of the great enhancements in Manageability is certainly the "Data Collector" in SQL Server 2008.  With the Data Collector you have the ability to capture performance metrics of your SQL Server(s) in a centralized way.  Not only does it capture performance counters, you also have the ability to capture profiler traces and output from your own T-SQL scripts. 

You can configure a Data Collector agent on all your servers and have them upload the information to a centralized MDW.  Where the Data Collector uploads is of course configurable so you can have multiple MDW's in your organization.  The great thing about this MDW is that you have historical data about performance.  This means you can go back to specific points in time for troubleshooting purposes and you have the abilitiy to do trending on the data.  We already had some nice features like the built-in reports in SSMS and the free Performance Dashboard but these were point-in-time reports and had little value if you needed to check a problem after it occurred.  The schedule of the collection and upload can be determined separately to spread the load.

We already provide you some reports that show you the power of the Data Collector and I have attached some screenshots to this post which are really worth looking at.  All these reports have drill-through functionality to give you more insight on what is/was happening on your server.

A little teaser but make sure to download the attached zip which contains more cool screenshots.

*EDIT*
There is a great post here made by the PFE team in the UK.