P: New user interface lacks contrast and many usability cues, lots of other problems

It's exactly as what Von Glitschka complains about Illustrator. It seems that Adobe developers add half baked features nobody wants instead of fixing bugs and that generally they don't use what they develop. Otherwise how would you explain releasing a software with such a huge bug as the Liquify one? Let alone the crappy interface...

Quick mask doesn't really show in the layers, before it has much better visibility.
Also, opacity and fill has lost it's quick slider.
That makes no sense, now it's way harder to work with Photoshop.

The new Photoshop GUI is genuinely problematical with its lack of contrast in all but the darkest themes. It's not a question of "getting used to a new UI," it's just plain too hard to read. I personally gravitate to the lightest theme because the dark theme tires my eyes during a long session, and I suspect I'm not alone in that.

If Adobe is going to insist upon being trendy by jumping on the flat-look bandwagon, at least give the user the option of determining how dark the text reads in relation to the background. If HTML is now being used for designing the interface, there's no reason a few additional customization options can't be thrown into the mix.

This mess is inexcusable. The Mac interface doesn't look like Mac software. The Windows interface doesn't look like Windows software. Adobe has gone to great lengths to piss off their users on both platforms. If that was the goal, then we can all say, "well done."

OK, I hope that now Adobe has got the point. If not, this will be the worst financial disaster for them over time. I've two points to add:

1. Following a suggestion given above by Greg Justice, I went into Time Machine and dug out Photoshop 20015 version 15.0.1. I have now side by side in the Applications folder PS 15.0.1 and PS 15.1. Tonight I will archive 15.1 to make sure it doesn't start up first. 15.0.1 is what I want, reasonable interface, quite decent. I vow that I will not update till I am pretty sure that their next update is back to something I can use.

2. The new version messes up the shortcut Ctl-tab that allows to cycle through windows and tabs! And no, it's not the Mac OS, it's the new Photoshop. If I try in succession both version, the shortcut works with 15.0.1 but not with 15.1, it is as simple as that.

As I always have 5 to 10 files open at the same time, this shortcut alone is reason enough not to use 15.1.

Get back to work guys. We're not a bad bunch, we will forgive you this misstep. If you roll it back.

Thanks Kris, that worked!
(The only downside so far is that I get an error when I try to load my Workspace. (No error name or error number tough.) And then the workspace seems to load anyway.)

Horrendous - simply horrendous. As with many others, I've used Photoshop professionally every day for years and you've just made my working life worse. Why?

The new Photoshop CC (2015.1/ 20151114.r.301 x64) interface is such a strain on the eyes. Why change it so drastically and create a learning curve? When you look at it, it just looks like something went wrong with the programming, as if its some kind of incomplete beta test.
Your users have work to do! This isn't a hobby for some of us. This is our livelihoods that you are tweaking with!
Also, I opted not to erase the previous version and it got erased anyway. Now I am scrambling to figure out how to downgrade when I should be doing work.

You can also revert back to the previous version from the Creative Cloud menu bar icon. Find Additional Apps > Previous Version > View Previous Versions

Yes, overwrite the whole folder with the old one from your time machine in the applications folder on your hardrive