Linux

Five years of Mate (which is essentially Gnome 2 on life support) replaced by a couple years of KDE Plasma.

Mate treated me well enough, it was mostly stable, capable, and competent. But it was a bit crusty around the edges, and being so niche meant search-engine-visible help resources for anything than went wrong were virtually nonexistent. In hindsight, using it as a beginner's DE was probably a mistake. I suppose in being so austere and devoid of resources it taught me to develop more of a "get to the bottom of it yourself" attitude to debugging and have humbler expectations about form versus function, but that's a pretty rough sell to most people. Mate is definitely better as a drink than a desktop environment.

I don't need to talk about KDE Plasma at all because the rest of the thread already has. I have nothing new to add beyond the comment that I like their mascot character.

I have no informed opinion on Gnome 3. All I've gleaned about it is that it's supposedly "my way or the highway" by design, and the "my way" in question is controversially counter-grain to a lot of established expectations (e.g. it's literally why Mate exists). Which is neither here nor there to me, objectively. But I will say I have no interest learning a new way of doing things, even if it's theoretically superior, when a conventional system still exists, is viable, is highly polished, and is kept sharp-edged. Hence, KDE Plasma.