Now that this is live, and I can't turn it off, I'll repeat my earlier disappointments. The left nav bar takes up entirely too much real estate for mostly useless links. I thought hiding the bar would help things, and now that it's live, I can see some problems with the approach.
Most of the left nav links are infrequently used. The "Questions" link, however, is probably my MOST frequently used link, and making that a two step process is really annoying.
Also, the "opt-out" for hiding the menu, and turning it into a hamburger, is sort of a secret. How is a new user supposed to know about this?? Even if a new user figures it out, the links that live under the hamburger are a mystery!! There isn't even a drop down on mouseover that would give someone not in on how the page works a clue about what's under there. A user should not have to know how a web site works to figure out how to use it.
So, where does that leave us?? I feel like I'm working with a UI that's significantly less functional than that UI I used to have. I'm not a web person, so I can't even pretend to understand (or, frankly, care) why the change was necessary.
The amazing thing is that just about all of the flaws were pointed out after about two minutes of beta!! This stuff should have been dealt with, instead of simply badly patched. It demonstrates not just a difference of opinions, but poor process.
As a last note, I'll add that this is SE, and the people here have a reputation of knowing what they're doing. I can't help but feel that a backward step like this hurts the brand.