KDE 5 Plasma is amazing. Subjectively, I feel the following about it: after over 2 decades of being extremely buggy and extremely ugly, KDE has became a culmination of desktop PC GUI design, the best I've experienced so far (tiling WMs can be better in certain tasks and settings but not universally).
Manjaro with recent KDE provides me what I call "a lot like Mac but way better" experience (surely, there are aspects in which Mac or any other system is better, I just don't know which particular).
And it has made a great replacement once Ubuntu lost its Unity (right at the moment it became polished enough).
The only disappointing part are the plasmoids - I could make a lot of use of some desktop widgets but it's pain to find useful plasmoids and make them work, making your own plasmoids seems complicated.
In fact I'd be comfortable with a generic terminal plasmoid (with a read-only mode option) and a generic browser (just display a specific URL, refresh periodically, basic HTML support is enough) plasmoid but it seems too much hassle to build them. I just wonder why these are not available OOTB.
More (and easy) hackability could also make KDE even cooler.
UPDATE (some more things I'd like to be fixed just came to my mind as I had a cup of tea):
KDE still has not reached the Unity level of the Mac-like global menu (D-BUS menu) support: some apps which worked with Unity global menu perfectly (and also work on Mac) still won't work with the global menu in KDE. I had to fall-back to the "Windows way" of having menus in every window to maintain uniform experience among different apps - too many apps left the global menu bar empty. This isn't a particularly serious problem though. Unity's facility of searching through menu items was also insanely cool and should be ported (as we have the D-BUS menu anyway).
I want to completely disable clipboard management so it wouldn't remember clipboard history (which is a security issue) and wouldn't clear the clipboard as soon as I close the window (extremely inconvenient - I often want to open a window just to copy something, close it and paste that somewhere else). But I could never find how.
I don't want tray messages to pop-up as soon as I start copying files. I'm not even sure I want them when the process finishes (perhaps there is a less annoying way to know it's done).
I'd like an option for Yakuake to actually replace Konsole so whenever I click "open terminal here" (preferably - in any app) it would open a new tab in Yakuake.
That's all the improvements to make KDE effectively perfect IMHO.