The main problem I see with left navigation is that I don't see anything of value in that left nav bar. I don't see anything I would ever actually need to click on. Yet this is being given priority over the right side, which I use all the time. In fact, the entire reason I've said I wanted responsive design was so that the right side bar never goes off the screen.
We're here to read questions and give answers (and vice versa). Not teams (whatever that even means), not jobs, and not to check out users. And, if I'm interested in a particular tag, I'll search for it. Everything in that left nav bar seems entirely useless to me.
My current plan is just to manually hide that bar if you force it on me. It seems to just be wasting space. I hope you will make this unnecessary by moving the stuff from the right into the left bar, if you're going to add it.