Be sure to read David Rutten's post for a more complete overview of the new features....
Click here to download the latest Work-In-Progress release of Grasshopper.
A few cool features that immediately come to mind...
1. You can now "Undo" and "Redo" (YES!)
2. You can now "Find" components being used within the definition. (Right-click to find the component... renamed components can also be found)
3. You can now enable/disable components.
4. There is a new color scheme for components making their 'state' more easily legible in larger definitions.
5. New user interface components: "Multidimensional Slider", Bar/Line graph visualization...
6. There are many new fun toys to play with in this release including Metaballs