This is an interesting interface, and one that will succeed because github’s target audience is already used to it. I wonder if it would work in other contexts as well. I suspect that in any situation where people use a web app all day, having a central location where you can do anything, rather than having to hunt around for a control panel. In Bret Victor’s now famous talk on editors, user interface, and what you should do with your life, he said that Alan Kay’s driving motivation was that modes were not good. I think that this is in many ways a reflection of this. On github, I generally needed to be on the right page in order to do this. Previously, the way that they fixed this was by making every action highly redundant. However, especially in a community that prizes keyboard shortcuts, the ability to use one key to focus the command bar, and then just type in a command, is much more powerful than clicking on buttons until I get to one of the pages that has a redundant command on it.
I think that this is one very clever approach to reducing the number of different modes you need in webapps, and I hope that we see more apps following github’s lead. However, I think that in a wide consumer base, it will not be viable for many years, and that we will need more tools to make the web less modal. I think that bookmarklets and extensions definitely help, if your app has simple actions, like tweet, or post, or bookmark, but it is still an unsolved problem.