The new features are:
The breakpoints and Firebug Lite additions are my favorite new features. But why did I include Firebug Lite when all web developers (should!) have Firebug installed? Because when code runs on the Playground, it runs in an iFrame. That iFrame does not have the Firebug object initialized in it. Since it is a cross-domain iFrame, there's no simple way to add Firebug to the iFrame's window object, so adding Firebug Lite was the best approach. This makes it so you can now use all of your favorite Firebug debugging convenience functions in the Playground!
To use Firebug Lite and breakpoints, simply click on the line number you want to add a breakpoint to and hit "Debug Code". This will insert Firebug Lite into the output and pause the execution on the breakpoint line number until you to click the play button to continue. Try adding a breakpoint to a line, clicking "Debug Code", then opening Firebug Lite and typing in a variable name to inspect the contents/value of the variable at that point in the code.
Adding breakpoints and forcing Firebug into a local function context were really fun engineering problems, so if you want to check them out (or contribute code to the Playground) go to the open source repository for it, come chat it up on IRC, or talk with me in person at the quickly approaching Google I/O conference (early bird registration runs until May 1).
Also, it's really important that you share your feedback so that I know what you'd like to see in the next version of the Playground! Thanks, and enjoy the Playground!