I would say information density was too high. All those always-on indicators: 3D scrollbars, buttons, etc. create a very busy picture. Today's interfaces are much cleaner which comes at a price of less information and hence, more ambiguity, but I for would rather pay that price than go back.
One problem I see is that while the UI itself has been simplified, incidental complexity has crept in other ways. Most importantly, the OSes themselves as software systems have clearly grown ponderous and unwieldy so that today they are more bugs and more of those bugs can be subtle and surprising. Also, there is less uniformity in UX across apps (and UI frameworks).