These are the key differences in the new version of App Inventor (App Inventor 2)
No more Java app for the blocks editor! Just click the Blocks button in the top right corner to edit the blocks for your app.
You can mouse over an event parameter or local variable definition to get to set and get blocks for that variable. No more need for the "My Definitions" folder!
You can copy and paste some blocks, then change things. In the above example, an event hanlder for MLKButton.Click was copy-pasted. Then the triangular widgets on the blocks were used to change the copy so that it works with MalcolmButton.
Many blocks, including 'if-else' and 'make list', provide a blue mutator button. When you click it, you can transform the block, e.g, adding 'else' branches to an if.
App Inventor now has local scope-- variables are no longer in the same global namespace.