Leaving the taskboard and backlog management out of it (which is really the #1 feature, but in order to be effective would require your team to transition to an iteration-based agile process), here are some of the big points for me:
- vastly better web interface (my team actually prefers it to VS itself when it comes to working with work items, partly because of the ability to use a larger font size)
- local workspaces. This gives you things like checking out, undo, etc. without being connected to TFS, and files are no longer marked read-only when they aren't checked out to you, and editing files using non-TFS-aware apps works properly. basically it makes TFS work like Subversion.
- multiple simultaneous gated check-ins (suuuper useful when you've got a big test suite and need to make sure your code base always has a 100% pass)
- a new "test adapter" API that makes it possible to run NUnit and other non-Microsoft test runners right in TF Build, and get test run results properly recorded with the rest of the build metadata. Code coverage results are supported, too.