You've put a lot of feedback in one answer. I'll go through and add comments in this manner to try to respond to each one.
The good
Finally a layout that works with window widths that aren't ridiculously large! Thank you! That alone makes the new theme an improvement, despite its flaws.
The bad
status-planned We are working on optimizing the break points. Most view ports have the same or larger line lengths for the main content area. Some are smaller. There is no perfect way around this, but we are looking at ways to make it better. This is complicated by the fact that changes here could significantly impact ad revenue on sites with paid ads.
As for the left nav, we are sticking with it and are focused on using this space more effectively. We are adding features there and will be looking at other improvements such as making it easier to hide/show without going to prefs.
Unfortunately there's a threshold effect. At a width of 816 pixels, the content is laid out in a reasonable manner.
But widen the window to 817 pixels, and suddenly there's this huge right-hand column which shrinks the content to a ridiculously small area!
The width of the useful content (the post text) has gone from 88% of the window width to 46%! How does this make any sense? The content should have priority over fluff.
This is with the left bar disabled (https://unix.stackexchange.com/users/preferences → “Hide left navigation”). That left bar contains a few mildly useful links, but these links don't need to be visible all the time, just on the page. Most of the left bar is wasted blank space, so the left bar does not make sense as part of the layout. The left bar should be replaced by a top row of links that scroll away. I'm aware that you're planning to put more links to question links there, but all that stuff still doesn't belong on the page permanently.
It sounds like this is an old bug. If it hasn't been made dramatically worse with the new theme, then it really should be brought up independently. → Difference between "foo" and “foo”, 'foo' and ‘foo’ — and -- as well
A design bug in the old layout was that some punctuation characters are mangled in question titles. Look at this title:
The meaning of a long dash? Ah, no, it's --, correctly spelled in the title, but mangled in the title, which means that it's mangled in search engine results too. Or look at this one:
Well, it won't work if you use “ and ”: only ASCII characters such as " have a special effect. Oh, wait, once again that's just because Stack Exchange mangled the title, so it looks wrong on the site and in search engine results. This bug already existed in the old design, and your response was
We looked at the quotes on the last several hundred posts. While a few would be more correct without fancy quotes, the vast majority are correct and would be negatively impacted by such a change. Given that, we want to stick with the net-positive of replacing the quotes globally.
Yeah, no, that doesn't make sense. Saying that replacing the quotes is “net-positive” doesn't make it so. It's a cosmetic improvement that can be done manually, versus mangling that affects the meaning and can't be bypassed. Can you please stop mangling punctuation in titles?
status-review We are happy to consider alternate ideas for the site header image. I encourage you to start a separate thread to generate ideas/options and we can work with you to make it happen. Or we can remove it completely. → What should our site's header graphic be?
The site image is really bad. Fortunately it's so faint to be almost invisible, but for those who can see it, it gives an awful image of the site. Here it is with a bit more contrast:
At first, when I couldn't really see it due to the poor contrast, I thought it looked like some circuit wiring diagram. Did I stray onto Electronics SE? Upon closer inspection, it contains some vaguely Unixy-things (a couple of punctuation characters that are meaningful in Unix shells), assembled in a way that screams “overly complicated”. The image doesn't need to assemble pieces in a way to make sense, but “overly complicated”? At best that's a designer who didn't do the research, at worst it's pretty offensive. ”Oh, Unix & Linux? Go there if you want to see some needlessly complex, incomprehensible stuff!“ That's a profound lack of respect of our community.
The ugly
status-declined We made it very clear from the beginning that we intended to remove this. It wasn't done out of lack of respect at all. It was done in an effort to unify the presentation of several important UX elements across the Q&A sites. After further review we have decided this is an area that we will keep standardized.
Speaking of lack of respect of our community, one of the few unique design elements of Unix.SE was the appearance of tags and some other links with a leading slash, leading to the habit of writing /foo to mean the tag foo. This is gone. Seeing as several sites had a somewhat distinctive appearance for tags ({foo}, /foo, 'foo, etc.), you could have kept that one column in the database. Removing that screams “it didn't exist on SO so we don't care”.
