THE GOOD:More cars, A Flyable helicopter, WAY better hideouts, AI improved somewhat, motorcycles & Scooters, more boats, more weapons, a lot more interiors. There's also a much wider selection of features in this game. Custom Soundtracks.
THE BAD:
You hold fewer weapons, gangs in the city are dumb, the city feels so empty, not much variaty of music(no classical or reggae station like in GTA3), you can't carry as much weapons, you loose life from tripping on a curb, too many dead ends, no tunnels, no trains, still many glitches, short less challenging game in general. Also though the city's borders may be big, the city areas really are small with most of the space taken up by the beach, the sea, and the airport.
Not to be mistaken, i think this game is great for many many reasons. A lot of the above I only mention I don't really care about. However, I have a feeling this game will get old faster than GTA3 did....