I will always miss Aqua. I remember vividly the first time I used it OS X on a new Bondi Blue iMac, with the soap-on-a-rope mouse. I walked away beaming from ear to ear that there was this gorgeous juicy, make-you-want-to-lick-it UI on top of a UNIX environment with bash and gcc and all. The future was bright! Later on, ZFS almost made it in; I envisioned that the UNIX world would quickly collect around ZFS as a standard filesystem format and all would be grand and the UNIX wars of the 90s would be a distant memory.
ZFS never made it. Ho-hum. Then, gradually, release by release, the character was drained out of the icons and buttons and all became flat, blithely following the fashion of the day. Now it doesn't stand for anything in particular. The irony is that the flatter icons on the newer releases render with far less compute power than the rounded ones of the older hardware.