I showed this once to a non-techie friend, and they were excited to use it. My little demo covered the difference between F and Shift+F to show how it is case sensitive, and I navigated through multiple pages without using a mouse. I assured them that they don't need to learn all of its commands, and they can open a cheat sheet on any page just by typing a question mark.

This non-disruptive introduction into Vimium made me think about my old Firefox setup. Back in the XUL days, I was hoarding dozens of add-ons that altered the Firefox UI to the extent that others could not use my browser anymore. They would click on a link and wonder why it is not working while my browser would amass many new tabs at a customized, designated place. The browser worked as I intended, but the extent of my customizations made it unusable for regular Firefox users.

Therefore I think that these new, more limited extension APIs offer an overall better experience for end-users. Maintaining usability for "regular users" should be the upper bound for the amount of customization an extension API provides. If it goes beyond that, it seems to get out of control.