The look and feel of OS 9 is still working today. And I wasn't a Mac user so no hard nostalgia feeling (compared to a Wintel box). Ironic that I was so into hyper fluid dynamic UI (css3 or other stacks) now I'm finding myself missing Win 3 / OS 9 "fixed function pipeline".
BeOS and OS 9 were well designed. I feel like we've sort of lost our way about what made those OSes good, replaced it with very “showy” graphic design. Operating systems should be a tool for information and communication, not a modern art exhibit.