While I'm happy to hear about this update and the beautiful responsiveness of the redesigned sites, I've got to disagree on some of your design choices. Some of these have already been covered by other answers, but I'm just stating my opinion here.
Why standardize everything? Things like fonts, badges, up/down vote arrows and custom banners are the best part of each SE site and make each one of them different and peculiar. The first time I took a look around I was amazed about the dedication in creating such sites with their own personality, and it would be a shame to lose it. Please, don't throw these details away! Don't make SE look like a boring collection of identical sites.
A non-collapsible left navigation bar for things like "Users", "Jobs" and "Tags"? Why? You know how many times a day I use those pages? Zero. I don't really remember the last time I visited one of those, so I don't understand why anyone would like to have all that space cut off to just hold useless links. Not to talk about the little space left for the actual body of the site, which looks suffocated by the presence of left and right bars. Looking at the GIF you guys provided as a demo of the responsiveness of the site made it even clearer: if I have a ~500px or ~1000px screen width there is no difference in the size of the actual body of the site, because on one hand the left bar is gone but the screen is narrow, on the other the screen is wider but the sidebar eats out the additional space. What am I supposed to do? Write my own user styles to get rid of those bars? I have a wide screen because I like to do more than one thing at a time, and use more than one program at a time, not because I want to use my browser full screen and have web pages fill all the existing space with content.
The top navigation bar looks way too clumsy on narrow displays. There are two million icons and a search bar the size of a grain of rice, plus a menu button which only contains the same left navigation bar you have on wider displays. Why collapse everything up to the point the top bar is just a mess full of icons? Why not move some of those in the menu instead? Icons like "Help Center" and "Other SE sites" are basically up there only to waste space. I personally clicked them maybe twice in my lifetime.
EDIT: looks like Joe already acknowledged this, and changes will be made to the currently proposed style.
