There are 3 playable main characters. Their names are Trevor, Franklin and
Michael.
You can switch between them at any point when not on a mission. When you switch, the camera zooms out into the sky and then back down to the character you switched to.
You can switch to another playable character during missions where they work together. This happens instantly.
Some missions will only have one of the main characters, whereas in other missions it may be two or all three together.
Each one has its own personality, motivations and skillset. When you're not controlling them, their lives continue and you may be surprised to see what situations they find themselves in when you switch back.
Each character has his own story arc.
Of the main storyline missions, there'll be several complex missions focussed on heists in style of "Three Leaf Clover" from GTAIV.
Some characters from GTAIV will make an appearance in V. Niko won't be one of them.
During research, Rockstar took over a quarter million photos this time!
Census information and vehicle sales data were used as a guide to depict the game world.
Big advances in the game's lighting model with vast increases and advances in draw distance and levels of detail.

You can flip off other characters in the game, and see interesting results dependent on who you're flipping off.
Vehicle physics have been updated. "The cars hold to the ground a bit better". It now feels more like a racing game, compared to GTA IV where they felt "big and boatlike".
After a hiatus in GTAIV, Ammu-Nation is back!
A lot of new side activities, including yoga, jet sking, base-jumping, tennis and golfing on a full golf course!
Rockstar felt that dating wouldn't fit in this game and have stripped this feature.
Mobile phone will return, but friends won't be calling as often as they did in GTAIV. It will be more streamlined and used for things like emails and Internet browsing.
Random character encounters throughout the map, much akin to those in GTAIV - but this time there's a greater variety and they're fully motion captured.
Dynamic missions, similar to Red Dead's "Unique World Events" make a return. Examples include characters with broken down cars, a hitch hiker, or two parked cars with dead bodies around them - which you can investigate to initiate dynamic missions.
Dynamic "state machines" were coded in the game, allowing non-playable characters to go about their daily lifes. For example, you may see gardeners and cleaners waiting at the bus stop in the morning, and then find them tending the gardens and houses in the game's fictional Beverly Hills later on in the day.

GTAV's game world is bigger than the worlds of Red Dead Redemption, San Andreas and GTA IV combined!
Fully explorable Mount Chiliad
A military base
Salton Sea region
Wine country
Wilderness
Fully explorable ocean floor along the coastline

Most vehicles in a GTA game to date, including - but not limited to:
BMX
Mountain Bikes
Road Bikes
Dirt bikes
Planes
Variety of cars and trucks
Helicopters
ATVs
Jet Skis

Michael

Michael is a retired bank robber, who made a deal with the FIB and is currently in the witness protection program.
Lives in a mansion in Rockford Hills (based on real-life Beverly Hills).
Married to wife Amanda, and has a teenage daughter named Tracy and teenage son Jimmy.

Trevor

Former military pilot
Lives in a trailer in Blaine County (desert).
Used to work a few bank jobs with Michael back in the day

Franklin

Early 20s, works at a luxury car dealership.
In the demo, he was driving around Vespucci Beach in his 9F (equivalent to Audi R8).