Put "Shared knowledge for the open web" on just one line. It will look less clumsy, plus there will be more content "above the fold." (More content above the fold would be great on other pages, too.)

Generally speaking, dealing with developer docs, "less is more" is almost always true. I love elaborate design, but here, visual clarity and speed are key. Finding things quickly is more important than beautiful fonts and graphics. Therefore, take an "un-design" approach. This iteration of MDN certainly is a step into the right direction.

I love Open Sans, but, at least on Windows, Arial is more legible (and doesn't have to be downloaded first.)

Menu fade effects are cool as long as I perceive them subliminally; 400ms (?) feels extremely sluggish.

As others have noted, page load times still need to improve considerably. In order to find my way around tech docs, I have to browse a lot; therefore, pages can't load fast enough. Grab a copy of "High Performance Web Sites" and heed the rules outlined there. 9 CSS files are not okay! >10 JS files are not okay! (Why would you even need that much code for simple docs pages?)