It's just... ugly. It's kind of a retro-ish, but chunky design. It really shows it full ugliness in the Finder windows, especially when you column view (which I default to). It looks like a design by committee mishmash of stuff that all collides. I'm not normally one to hate on stylistic choices in an OS. I've never had much interest in "ricing" my environment, but I'm really just not a fan of this.
All the individual little "bubbles" of things instead of buttons, or even just borderless areas denoted by an icon. The way the left column looks like it's just plopped down on top of the rest of the content. Like it's a separate entity, but it's not. Especially noticeable in how the lower cookie crumb trail at the bottom of the window just abruptly ends at the border to the left column. Also, the color choice for the sliders is... bold. Almost the most noticeable thing in the screenshot! This looks like a Linux Enlighenment DE theme from the early `00s trying to emulate OS-X. The dark view looks slightly better.
About the only positive thing I can find is the tolerances around the edges of windows for resizing feels more forgiving than in the past & w/ other operating systems (ahem: Windows). On some OSs/desktop environments, it seems like there's a 1-2 pixel area where the cursor changes and you can resize a window. In contrast, this works pretty good in Tahoe.
Forgot the Linux home server that was turned on and forgotten about but has been running continuously for decades of uptime and continues running off solar power even after civilization has ended.