Hang on, nobody has mentioned anything about what will be in it!
Firstly the city is much larger than liberty city was in GTA3. (More resemblance of New York -Times square, empire state building, Statue of "liberty" etc...) I bet the statue is called "The statue of Liberty City" or something though. -Im basing this on the fact that the layout of the city is completely different.
Secondly, there could very well be a fully operational Rollercoaster, "Screamer", featured in the game. Should be good if it was ridable.
Peds interactions have dramatically improved, with smerks and even waving at people while driving. Paths have also been improved (birds too).
Water physics has been improved drastically, and a (working) ferry port is featured. Shadows and Lighting has also been improved.
More realistic cars instead of the usual fantasy cars. Also features telecabinas (dont know what it is in english).