As I understand, the titlebar is not only providing information what page we are currently on, but also the path to the given option. Since Plasma has a lot of options and layers which we don't have access to from the same point, it's good to know that the visible UI is a sub-layer of the other option. Also, not all themes make it clear which option is marked, so even with the first tier window it's good to have some title above.
So all in all, I'm for having titlebar.

Ah, right. Till now I didn't see much difference between the chosen theme and breeze. However, now the shadow is different and indeed Breeze has subtle, soft shadows so this is not a Breeze issue. Sorry, for the confusion.

I updated to Plasma 5.17 last night and the first thing I noticed were... huge, ugly shadows. This was a surprise. It's something different to see shadows in the screens and I did find them too big, but seeing them in a live system is something different.

Can you make that shadow softer? The thinner it is, the harder it looks, thus less "shadowy" effect.

Hmm... Maybe something in between? The left proposal is as if there was almost no shadow. It looks nice, very flat, but a bit overboard? It's not bad either. I mean, both look OK in their own way. It's probably a matter of personal taste on this point and I wouldn't be angry if one or the other was chosen.

In T10891#204217, @niccolove wrote:

On the right, there's a bit less sparse shadow with a bit more transparent background (personal favourite).

Kvantum theme allows for turning the grouping of taskbar icons on and off so this should be possible on Breeze as well. I found the grouping to be more aesthetically pleasing. I'm not sure if such grouping is already added to the Breeze theme thou. There are already separators in settings so we can define which icons go into which group so basically this is all well covered if the grouping is available.

I like the general direction where this is going. I'm using Kvantum theme on a daily basis which adds many of such effects anyway so I know from experience how well it works.

I love this whitespace/expanding whitespace idea. It looks like I have a very similar sense of aesthetics as Nate so I'm agreeing with all his posts above. Kvantum themes can group toolbar buttons already and then the screenshots usually make a huge impression because with so many changes the whole Plasma feel is strongly altered, so people complaining about KDE look or those preferring Gtk look are interested in it, because it finally looks more in line what they are expecting.

I second to the white background on checkboxes. The white background suggests it's clickable, active. Gray or the same background as the other window suggests as if it was inactive for some reason.

When we're at the possibility of "how to restore default panels when someone messes up things or remove all panels", why not add the option of restoring desktop/panel defaults to the desktop context menu? Of course, there would have to be confirmation pop-up (like: do you really want to do it, this action is irreversible and will bring your desktop space to default setup).

Plasma has separate touchscreen settings (edges) so maybe this could utilize some missing desktop features instead the desktop toolbox? I don't use touchscreens but is there really no right-click possible on them? I find it hard to believe. What about long push or double-finger tap?

I use Activities and I have no idea what would I need the Desktop Toolbox for. I disabled it right from the start and never miss it. It's one of the things that you are ashamed of when showing Plasma desktop to others and quickly show how to get rid of because it automatically creates friction: what the hell is it doing on my desktop? Nobody uses it and no other DE has it - for a good reason. Desktop Toolbar is like an ugly mole on the desktop's face so to speak, definitely not a beauty mark...