Don't get me wrong, i'm excited about this game just as you all are but lets face it, GTA III was a revolution and this will be smaller scale revolution. I will quote hmnut7 here. He explains it in a very good way.

I think it will be a HUGE leap, but still not as big as the leap from GTA2 to GTA3.

The leap from GTA 2 to GTA 3 was the jump from 2D to 3D, the jump from making a square to making a cube.

The jump from GTA 3 (and it's fellows) to GTA 4 is something like the jump from 3D to 4D, from making a cube to making a Tesseract (cube with a cube within the cube).

Instead of making the city bigger they are making the city more complex like a tesseract is a more complex cube, which is a huge jump. But again not the same as the jump from 2D to 3D, because once you have made the cube, even making it more complex is not as big revolution as the making it in the first place.

There, i could not have said better.