Now I have yet a separate terminal, a separate file manager, a separate Python REPL, ... How is this any better than just keeping those applications open and switching between them?
I really hope this isn't just a poorly-implemented window manager inside a web browser.
For other plugins there is very nice integration so that you can have interactions in one plugin update outputs in another and vice-versa.
The ease of creating a connected ecosystem of web-applications connected to your workflow is one of the powerful aspects of the new platform.
What would great integration look like to you?
For me, this means using emacs-ipython-notebook. This brings the ability to interact with a running IPython Kernel into the text editor that I use daily. It's the best of both worlds: interactive plotting+notebooks+rapid development assistance from IPython, and thanks to Emacs, all the keybindings are what I expect and I'm already familiar with how to extend the environment to suit my needs. EIN has been transformative for me.
Perhaps my needs don't match the needs of others. For most people, I imagine the Jupyter-lab style of environment that's self-contained in a browser would in fact be a step up from the ad-hoc notepad++ windows and PuTTY sessions used previously.