-Just like acquired cars, upgraded guns are now a permanent feature in your arsenal. So don't worry about losing your weapons.
-Players can call in laser guided air-strikes at all times, the explosion disintegrates enemies. Players can also call in a Predator drone, the view becomes top down and you can pepper the area with bombs, or use a smart bomb which you guide with the anologue sticks.
-cars now deform more realistically
-Remote control gun shoots a bug that allows the player to control any NPC vehicle much like as if the player was driving it themselves. Hilarity ensues with this tool, for example you can control an enemy gangs car and purposefully ram into the police, watching the confused driver get chased by the cops is hilarious.
-Meleeing with a weapon close to an enemy = LOW BLOW! NUT JAB! Stiff jabs to the crotch all round.
-LB button is a sprint button, and also a modifier button, it amplifies almost anything the player does, for example sprint towards a car and attempt to carjack it, instead of opening the door and pulling out the driver, the player can performs a missile drop-kick right into the drivers seat, run from the side and you'll break through the window and the passenger goes flying out the other side...awesome stuff.
-If you prefer to carjack normally, the camera now zooms in as the player shoots the passenger in the face at point blank range, or repeatedly bashes their head with the doors.
-dedicated grenade button. Very nice.
-No more eating to regain health, health now regens. very reasonable for a game like SR.
There are also some RPG elements, with perks and other upgrades. What used to be Respect is now XP.