This looks beautiful, and I'm definitely in love with the idea of writing presentations in HTML/JS/CSS rather than in a blob of Keynote or Powerpoint, but there's one thing keeping me back:

Keynote gives me a second display with a timer, the current and next slides, and notes. That's huge. On the rare occasion that I present I don't script out everything I'm going to say, and the presenter info keeps me from rambling too long or forgetting what my next point is.