Am I the only one who finds the "evaluation as conversation" idea incredibly annoying? I mean, it's not like I can't run the conversations 100x times faster in my head and with less "visual noise". And teaching you to run code inside your head should be the main purpose of such exercises - outsourcing to an interpreter you have a conversation with defeats this purpose!
If I had been asked this question earlier, I might have said "It could be useful". Having tried this page, though, I agree with you. It was painful and distracting to step through the first example, and I can't imagine doing it for non-trivial pieces of code. At least for me, it would devolve into clicking "Next" blindly until the end.
A minor critique: to click 'Next' repeatedly requires you to move the mouse again and again to wherever it's repositioned. If you move it to the side, you can avoid this and let people read through as fast as they can click (i.e. much faster).
As a fan of both Little Schemer and The Hitchhicker's guide to the galaxy, I find this awesome. One suggestion, it be great to have parens matching in the editor/textarea
