Well, there are a few things. Glitches have been minimalised, but I suppose that's not graphics. A big change is the cars. The textures are alot smoother, and cleaner. Draw distance has been maximised.
One thing I know you would have noticed are the buildings. How on Vice City there were alot more "cleaner" buildings, yet on this one, the doors aren't clean, the windows are hardly see through if at all. But the thing is, they wanted to blend everything in to make it all look like it belongs together. In Vice City, the Well-Stacked Pizzas, etc, all stuck out and looked like they didn't belong, along with a few other buildings.
Of course, making every building look clean would present alot of loading problems, so they opted for the safer option and went with the classic GTA building look from GTA 3.
It's not the best looking game in the world, but it's the GTA world, and it's just fine in my books.