1. UML: Now draw UML diagram right from your Visual Studio. No need to buy another tool or go to Visio to do that.
2. Application Architecture Re-engineering: You have a ready made code-solution, now you have to explain it to some one but have documents. VS2010 will help you to draw an application architecture using you application code. This way you get a big picture of your application and also all linkages between different classes. Good for new joiners in an existing team.
3. Stuff for Manual Testers: Manual tester can breathe a lot better with VS2010 tools, log in all your test cases and select the test case to run and click record and then execute your test case as usual and see what you get. If test case passes, you get evidence (most clients in service based companies want that) as videos or if the test case fails, you get the steps to reproduce as videos. Cool, isn’t it?
4. Besides these a lot more in Unit Testing side.
5. A lot more features and integration with SharePoint.
6. Above all brings in .NET 4.0