One of the main things is that it continues to perpetuate primarily mouse-driven interaction with the development environment. Even when tools like this enable Emacs or vi key configurations, the integration just never quite works, and there are environment-specific options you are required to select that come from e.g. drop-down menus, etc. Interacting with UI elements is horrendously unproductive and disruptive to thinking. Putting it in the browser makes this worse, because then you've also got the browser's own key configurations, like tab switching or bookmarking, to worry about.

There is seldom any value in looking directly at code and at a console at the same time. But if you really want that, it's super easy to do it with a window manager like xmonad, or even just arranging shell windows on your desktop so that you can alt-tab between them easily.

You often want to quickly spawn and kill shell tabs, which themselves may or may not be in the same language. For example, I often have a tab in which I'm using IPython, another tab in which it's the working directory so that I can execute things with Python directly, mv/cp files, ls, etc. And then still more tabs in which I have background processes that check for file changes and run my unit tests whenever things change, sometimes a tab for Python 2 and another tab for Python 3. And further, development projects are almost always cross-language, so I tend to also have some tab opened for writing and working with C or Haskell at the same time, and possibly another with a psql shell.

Since there are so many necessary tabs just to do even the tiniest things, it means that any and all visual overhead must die. Switching these tabs, even if it is quick, inside of a clunky GUI application like a browser is just too unproductive -- the browser periphery already wastes maybe 5% of the available visual space, and then the overhead for the tab icons, clickable close buttons, etc., wastes another 5% inside of that, and then the width of the tabs is restricted because there's some left panel with directory information or in-memory workspace information (both total wastes of time), and the height is restricted below by some worse-than-plain-shell console; it just makes no sense. It's too much visual clutter and too inefficient to facilitate switching around as much as is necessary.

I think it should be emphasized again that the left-side panel showing either directory structure or the contents of an in-memory working environment are huge wastes of time. If you need to visualize a directory structure, that should just be another buffer, like a code source file, and when you want to view that you just switch to that buffer. There's no benefit to having some of your visual field distracted by it when you're working on other files. And the in-memory information is also generally crappy. It's another thing where if you really need it then it should just be in another buffer and you should quickly go to that buffer and give it all your attention for the short time you need it, then go back. It serves nothing by having it as an ever-present visual distraction. But more than that, relying on inspecting variables that way is a very infantile thing, and I see it a lot with MATLAB programmers. Their form of debugging is not to scientifically inspect and control the execution of the code, and use proper breakpoints and watchpoints to tells them what's going on, but instead to just "run everything" and then go and click to open up a spreadsheet-like view of a matrix variable or something and manually (!) inspect the data. Then they become reliant on this as a crutch and complain when it's no longer there, instead of learning proper ways to write tests and proper debugger usage and let those things automate the problem of zooming in on outlier data, messy data, or bugs.

Anyway, there's plenty more to say, but it's probably long-winded enough.