Linux

Beauty is in the eye of the beholder, and though I'm not a fan of GNOME since 2008 (I guess you can check my recent comments...) I concede you can see the (visual) direction they're trying to follow with Adwaita UI and trying to make it cohesive and coherent. Something I wish other DEs did that religiously, like Xfce, Enlightenment, even KDE itself, LXQt or you-name-it.

Still I think the problem with Adwaita is not that it's ugly or something (I'd say more that it is highly opinionated, as it has become the full GNOME experience - either you like it as it is and it fits you like a glove, or you have to use something else because there's no point in between), but a couple things even worse than that - (1) the serious issues it has brought to accessibility, i.e. not being able to tell with full certainty what is a button and what it is not in a toolbar, and (2) doing awful things in usability and UX for the sake of "convergence". Like putting the primary action ("open" or "save" buttons) of dialogs in the exact same spot where you'd find the close button in every else window. Why is that? Yes, because "convergence". On desktop.

All in all the hate towards Adwaita could be that it's allegedly a visible symptom of how GNOME has so much power over GTK that Xfce and co are doing black magic trying to get rid of it for their development. I've just read rumors so don't quote me on this, but I'd believe it can be true.