Bluecurve was amazing. For me, nothing in the Linux world has equaled it since. It was so nice and polished with thoughtful colors and design. It was quite similar in colors to Mac OS 8 & 9 (the platinum gray, the purple-ish blue).
I sort of agree with that. That was the peak Linux GUI we saw (at least so far). Sadly after the Gnome GTK2 vs GTK3 forks, none of the child products carried the same finesse as Gnome 2.4/Bluecurve. They were sharp, colorful and vibrant. Windows icons seemed more 'classic' versus the KDE-inspired Bluecurve, Clearlooks which were more playful.
Reminds me a bit of BeOS as well.
It's the icons angle and that thin black border around images, I believe