One of the things i believe felt artificial in games like Baldur's Gate II was how all the people in the town was standing still (except for moving a few feet once in a while). In a big city I expect there to be much more people in the streets and i expect most of them to be moving from some point to another.
So my idea was to always have a random amount of people walking (maybe some even running) around from one end of the party's field of vision to the other end, possible stopping a minute at the local merchant. This would really make the big cities feel authentic for me.
Still I know this might take some work and compared to the gain it might not be worth it. So what do people think?