Well one big difference is that once the load time for sims 3 was over you were in game and didn't face another load time unless going abroad or to uni. :)
Yes the load times for TS4 seem very short, but remember that this is just the base game not the game and several Eps, and also we have no idea what the computer specs were for those demos. Additionally by the sounds of it players are going to face a lot of load times, so although they seem short it will soon add up to several minutes if you want to visit several lots during a sims day.
