The complaints were that the UI was not customisable, with limited functionality, stubbornly did things the Mac way even if that was confusing to Windows users. The implementation was often slow and sometimes buggy.
But it had taste and attention to detail. It followed Apple's own HIG (design guidelines). The UI had some flashy details, but they had a purpose or at least didn't get in the way. I don't feel any of this in the current Apple designs.
UI was simple, maybe too simple, but it made sense. The only complaint from that era was the thin scroll bars and the flipped scroll direction, both of which are forgotten about and accepted nowadays.
As someone whose primary interaction with her computer is via her drawing tablet stylus I still loathe the thin scroll bars; I've turned them off in prefs but I still regularly encounter apps and websites that force them and they have to be really special to overcome that.
