Low transparency mode has this weird bug where the corner of rounded rectangles are black rather than fully transparent. Try enabling it in accessibility and then changing the volume.
Edit: I wonder why I am being downvoted. UI/UX bugs are bugs as well and need to be addressed!
That's a personal opinion and really doesn't matter on HN.
The link you provide is not about aesthetics, but about UX. So to back up your (pointless) claim that Yosemite is ugly to look at, you link to an article that rightly points out useability problems. This is like saying "The top speed of my car is too low, here's an article that talks about its slow acceleration"
I am assuming you are being downvoted because your claim is irrelevant to HN discussion and your link is irrelevant to your claim.
Your second claim is partially true though. While volume indicator does show black corners, there are plenty of rounded corners that do show correctly in Yosemite with transparency turned off.
Lastly, according to the HN guidelines, you are kindly asked to "Resist complaining about being downmodded. It never does any good, and it makes boring reading. "
I've been using it since 10.1, and the only time I thought they took a step back was with Mavericks. I attribute that to major UI changes needing a version to sort out.
It might look great, and I agree it's pretty slick and much more à la page if you will, but it's much less usable. Windows 8 looks so much better than 7 [1], but it's a train wreck in terms of discoverability and usability.
[1] one might argue that this flat UI trend originated at Microsoft that Apple is copying them and that some of the things Microsoft is doing now look much better than their Apple counterpart especially on mobile. I can't comment on usability, because I haven't tried either.