Probably the biggest difference was Display PostScript->Quartz. That and the UI makeover made for about a 2 year delay before OS X was released. From Bill Gate

Probably the biggest difference was Display PostScript->Quartz. That and the UI makeover made for about a 2 year delay before OS X was released. From Bill Gates' perspective, MS Office was a straight-forward port to Carbon, so I suspect he misunderstood when hearing it was 'mostly the same'.