Also, it adds more strategy. Now, there are long-term consequences of building science vs gold infrastructure. In civIV, you just had to change a slider to completely change the focus of your economy. Not only is this actually quite simplistic, it's not realistic.
In civV, you have to worry about building infrastructure (production is worth more and the buildings have costs), social policies (commerce vs rationalism), improvements (trading posts vs farms), and etc. All of this makes the way you run your economy have much more lasting effects. It is now very easy to overextend your science economy, crash your gold, and all of a sudden you can't build anything and can't support any military units. In IV, all you would have to do is bump the slider back, and it's magically fixed.
Also, by decoupling gold from science, it is now possible to get a large gold surplus. Since they've added so much stuff that you can do with gold, this adds another layer to the economy that didn't exist in IV, because gold was useless (you never had any and you couldn't do anything with it).