I'd place the high water mark a bit earlier, probably around 10.6. They mostly hadn't started removing or dumbing down stuff yet: you still had Front Row, because they weren't trying to sell you Apple TV; it was new enough to have the modern Cocoa Finder but also still provided Rosetta; iOS influences weren't a problem yet; iCloud wasn't so deeply integrated and File menus still showed Save As by default; multi-monitor use hadn't yet been ruined by the new full-screen app modes; you still had real scrollbars and the OS didn't make the mistake of treating indirect touchpad scrolling the same as direct touchscreen scrolling.

There's still a lot to like about newer versions, but the stuff that was removed on the way to 10.6 I was mostly glad to see go (excessive brushed metal) or at least don't miss now, while there are some things from 10.6 that are now gone and I still miss them.

I don't see the removal of ObjC GC as all that bad, especially in comparison to the death of the Cocoa-Java bridge. At least the former offered a compelling migration path for actively maintained software, in the form of ARC.