GameInformer have been quick to post up a couple of new articles on some of GTAV's gameplay mechanics, shortly after today's gameplay trailer debuted. The articles focus on the specifics of Gun Combat and Customisation. Game Informer were also nice enough to give us 7 new screenshots! Summary of new info is after the screens.
 

 
Gun Combat

• Animations, targeting and camerawork has been completely overhauled to open up new opportunities for players in gunfights
• Players can now run and shoot comfortably
• When you fire your weapon, your character's movement will change from a casual walk to a combat jog, giving you greater mobility
• Gun play is still GTA-based, but aspects of RDR and Max Payne 3 such as targeting and transitions have been brought in to improve it
• Targeting system now has 3 distinct modes: assisted aiming, traditional GTA, and free aim
• Players can get out of a car in such a way to stay low and use it as cover

Customisation

• Ammunation returns, and now lets you customise your weapon with a range of improvements: extended clips, grips, colour tints, flashlights, scopes, and suppressors.
• Vehicle customisation has been taken a long way since SA and isn't just for showing off; some missions will require vehicles to be customised in a certain way
• There are more than 1,000 modifications in total
• Cosmetic modifications include: body kits, paint jobs, wheels, exhaust tips, window tints, tire smoke colour, bull-bars, extra lights, and lots of other extra accessories
• Gameplay affecting modifications include: brakes, engine tuning, suspension, horns, bulletproof tires, panel armour, and general repair
• You can take a picture of your finished vehicle using your in-game mobile phone and upload it to Social Club