To me the biggest improvement that 2000 makes to the 9x look are the move to a lighter base gray, as well as changing the default desktop color from that funky teal to a much nicer blue. As minor as those two things may seem they made the OS feel more pleasant.
But that may just be the classic Mac OS fanboy in me speaking. Mac OS never used particularly dark grays, so the almost-perfectly-middle-gray seen in 95/98 felt almost depressing in comparison.
95 copied more the NeXT than the Mac look. The grey is much closer, as are the button corners, an explicit rejection of the Windows 3 rounded corners. The NeXT greys made sense for the original cube, which was limited to black, white and two greys.
But that may just be the classic Mac OS fanboy in me speaking. Mac OS never used particularly dark grays, so the almost-perfectly-middle-gray seen in 95/98 felt almost depressing in comparison.