First: JupyterLab is definitely still alpha-level software, and is undergoing very rapid iteration.
I notice that a lot of comments are missing one of the most important ideas behind JupyterLab. It's built on a very flexible plugin framework (provided by PhosphorJS), and all components are provided as plugins. JupyterLab essentially is a reference set of components, but we really see people creating their own components, and a rich 3rd party ecosystem. This also sets a foundation for exploring other ideas for what a web notebook might look like, etc, and provides an easy way to distribute such a component. More than just extending via components, we've paid a lot of attention to getting having a good keyboard shortcut system, a good command/command palette system, etc. The video of the talk shows some simple examples of how JupyterLab can be extended: https://www.youtube.com/watch?v=Ejh0ftSjk6g&list=PLYx7XA2nY5....
Also, I see several people talking about having a UI that is really focused on notebooks. We definitely plan on an easy way to maximize a single pane (which might be a notebook, etc.) to be able to focus on one thing.