feature-requeststatus-completed (see Meta Stack Overflow)
My first impression of the left nav was pretty negative. All I saw there were links I didn't care about, and any time I went to a question list, the first thing I saw on every load was the navbar, and as I was reading through the questions, my focus kept switching to the new navbar on the left. I'm sure a lot of that was because it was new, but it kept happening, and it was frustrating. Disabling it made me feel better about it.
Now, however, after discussing it for a bit, testing it, messing around with it, I do think the only real problem I had with it was I felt like it was distracting me, pulling my attention away from what I was looking for. I found that one small change made all the difference for me, and that was giving the body of the page a background color. It didn't matter what color it was, as long as it put the primary content of the page into a separate box from the navbar on the left, it no longer distracted me. Here I just pulled the background from the header and added missing borders.
Image may be NSFW.
Clik here to view.
I can of course make this change myself later via a userscript if it doesn't go live, but that's my suggestion.