3D view
GTA3-era weapons (minigun, flamethrower etc.)
Ability to rob stores with your fellow Triads
Vehicle customisation
Auto-aim
Cover system from GTA IV (maybe slightly altered)
Side missions (taxi missions, vigilante missions, paramedic missions)
Easter eggs
New animations
Garages (fuck the yellow parking spots in GTA IV)
Better AI
Spike strips
Multiplayer (DS Download Play FTW!)
Variable weather
Empires
Cockpit view
Game is rendered in real-time (or according to the settings of your DS / DS Lite)
Submersive vehicles
Better melee weapons (we're all tired of the baseball bat, I want crowbars, police sticks etc.)
Plant car bombs
Hide areas