I'm a fan of this tutorial, thank you for it. Two suggestions for the Reshaping Data page.
Motivate the melting example (or change it so it is motivated). That is, what insight does the melted view communicate? Most of your examples are helpful because the reader understands why it was done and what is gained.
On the Transpose, state what happens when a key column is included in the transpose.
I'm a fan of this tutorial, thank you for it. Two suggestions for the Reshaping Data page.