Being able to watch how I write. It's like seeing a photograph of oneself for the first time. I had no idea how often I restarted sentences, for example.
There's other stuff I want to see that should be even more interesting. When it's replaying, I want to be able to see characters that make it into the final version in one color, and others in another. Then I'll be able to tell when I'm writing well and when I'm writing badly.