There are lots of things. One that comes immediately to mind is the fact that if you apply any Linux desktop “themes” you are actually pretty likely to end up in situations where certain software is hard to use because they have components or cursors or some weird stuff that was never tested with a variety of desktop themes, so parts of them end up with black text on a #020202 background or buttons drawn with a hardcoded background color or whatever. Since there is no actual cohesive theming environment, the splintered miasma of theming environments means there is effectively none that’s useful. This is, unfortunately, the nature of guerilla software development, and the reason why, as awesome as it is, it’s never taking over the desktop.
It's entirely subjective. For me, most Linux UIs have a charm that I'm not seeing in Apple or Microsoft UIs. Those are often "too clean" or "too polished" for me. I'm not saying that font issues shouldn't be fixed, but it doesn't bother me that much. For what's it worth, I also really liked the aesthetic of macOS pre-X.
Linux is perfectly capable of good font rendering. A lot of people dont care/cant tell and show off screenshots in which they are intending to show off their theme/backgrounds/colors and are only unintentionally show off the fact that their choice of fonts and settings they tweaked for no reason is subjectively or objectively worse than default.
For me, in Ubuntu LTS, Gnome looks nice and has good font rendering. File drag and drop doesn't work, I can't use their default file manager like I use Windows Explorer.
So, not always the same thing, but always something.