I'm not using plasma5 yet, but I use KDE 4.x most of the time (except I do also have a Windows machine at work for Outlook and Visio and Windows VM on my linux laptop in case I need Visio away from the office).
"I'd like to be able to sort the songs per artist, album, etc in JuK, and for it to have a working Manage Folder dialog."
I've settled on using Clementine (I believe a pott of Amarok 1.x to KDE4/Qt5), but didn't test this specific use case.
"Adding support for PTP cameras (you know, most of them), that would really be great. Means I would no longer have to connect them to either my GNOME or LXDE laptop to then transfer the photos over the network."
My wife and I use Digikam for most of our photo organising and editing, and it downloads from my 500D just fine, had lens correction profiles for all my lenses. Raw support could be better (there is some support and even though rawtherapee seems to have more features and sliders the results from digikam's 'local contrast' feature are quite similar). Digikam 5 (the port from KDE 4 to Qt 5) was just released with lots of nice featutes (mysql support returns, new instagram-like filters, many others) and binaries for Windows and Mac OS too (made easier with KDE5's reduced inter-dependencies).
Sure, the bugs in apps that are part of the KDE apps collection itself should be fixed, but that doesn't mean you should have to use GTK or gnome-based apps when thete are better KDE-based ones.
You do know that you don't need to use a different computer to run GNOME-based apps, right? Well, it's not as usable with GTK-3 apps with their stupid menus thst don't work right with other window managers, but that's a GNOME/GTK design flaw.