The new code browsing features are another win. Once you've learned to use Ctrl-Scroll to zoom in and out on the screen, you don't even think about it. Once you've learned the Navigate To shortcut (Ctrl-Comma) and the Call Hierarchy navigation, you'll wonder how you got around your projects before.

VS2010 has much improved thread debugging; given the increased support for multithreading and parallelism, this is a necessity. Toolboxes and IntelliSense are now sensitive to the Framework version of your target project, so you don't have to wait until compile or runtime to discover that you've used a feature unsupported by your target. Again, given the increased number of possible targets, this is a necessity.

Visual Studio has supported test-driven development for years, but it was always awkward to create new stubs and to synchronize IntelliSense if you actually wrote the tests first. Now you can switch into consume-first mode to keep IntelliSense from running amok, and you can generate stubs from their usage.