I actually like the idea of the left menu bar, especially with modern desktop/widescreen displays, but agree it should be collapsible. It's a waste of real estate the moment someone uses more than one window (or does something nutty like this) - which is someone working on a problem is going to do. That's an extreme case, but it gets worse if say, someone's snapping 4 windows to a screen or more.
That said, this feels like a great chance to flatten out navigation to the other parts of the site - things like meta and chat feel like a more natural fit for the new side bar - and it's worth considering if they could go there, especially with sites without teams/channels. It's kind of odd to have to go to a little icon in the corner, meant for site switching, for things integral to the site.
And while I do get why getting rid of as much site customisation as possible - I do feel that those, badges and icons go a long way towards 'distinctiveness' - and these are important to people. While from a technical perspective - it's probably simpler, I think the special badges, icons and such are worth keeping from the perspective of the community. Changes should, hopefully add on to, rather than delete things that folks are used to.
Also, once these changes are decided on (I do hope some of the input here's taken!) could we have an "official" walkthrough of them, like I did for the 2017 new top bar on MSU? It's a much more significant set of changes than the top bar, and there will be many people going EEEK!