I miss Dolphin badly whenever I'm on non-free operating systems, even though I generally enjoy file management via the terminal as a fallback. In Plasma, I'm much more likely to do a bit of GUI file management than under any other circumstances.

'Default' KDE apps are often so well thought-out and complete that I never feel the need to deviate from them, and it's not unusual for me to install them on other operating systems when possible. I feel this way about Dolphin, Okular, Ark, Kate, Gwenview, Klipper, and Konsole/Yakuake, too (even though there are several great new terminal emulators out nowadays). And KWin! God, KWin's configurability is so good and it has some really killer compositor effects for productivity that are still unmatched.