Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Someone claiming to be a QA tester for Grand Theft Auto V recently blasted the GTAForums with a whole shitload of leaked info on the game. The post was deleted by moderators lickety-split, but the information has since been compiled into a lengthy list and posted on Gameranx. No word on whether this stuff is accurate, but it all sounds pretty plausible.
Mountain ranges cannot be passed, however they can be scaled almost to the very top. GTA V is "landlocked" just like RDR.
Los Santos is the only major city in the game. However, this doesn't mean there isn't a lot to explore; Los Santos is ~50% larger than Liberty City, with a handful of smaller cities and towns beyond the countryside. One of the larger non-LS cities is based on San Bernardino, another is Tucson, both are about the size of Alderney.
The world is definitely not going to be too small for people's tastes; Expect to see snow, deserts, and much more.
The city of Los Santos alone is larger than GTA IV combined. Los Santos is the largest and most important city. Unlike San Andreas, players will not be moving from one city to the next, doing missions restricted to that specific location. They will be structured in a way that's unlike what has been done in any previous Grand Theft Auto game.
There are at least two cities apart from Los Santos which are the size of Alderney; They do not share a border with Los Santos or each other.
The Italian Mafia will not be making an appearance this time around.
Los Santos alone is the same size as III era San Andreas.
The mountain ranges extend above the maximum altitude for player-controlled planes, therefore making it impossible to fly over them.
Los Santos alone will feature three airports (more are outside of the city), with the world itself being more than twice the size of RDR.
Ammunition will be making a return, however it is inaccessible; The use of underground arms dealers will be used instead. Shooting ranges will now make a reappearance as well, with the minigame being revamped and much more fun.
Every plane shown in the trailer (excluding the commercial passenger jets) is pilotable. No big planes (ex. AT-400) will be present.
NPC Passenger jets fly higher than the vertical limit of planes piloted by the player.
There are 5 counties in the game, with Los Santos being the largest of them all. Overall, Los Santos is 1/3 of the entire map, with the map being over twice the size of RDR; This can be compared to New Austin and Neuvo Paraiso being one huge city, paved end to end.
Bowling will be making a return, which will also be tied into a few missions.
"ENEMA" is a designer clothes label in the game, with the real-life equivalent being MANGO.
Seasons change in-game, with the main story taking place over the course of more than a year.
Safehouses will not be customizable, however they can be customized to spawn with different cars, helicopters, boats, and planes.
Weapons will be customizable.
Car customization will be making a return.
Character clothing can be changed, but the overall appearance of the character can not be changed. There will be more than three clothing stores to choose from, however it is not a major component of the storyline and therefore optional.
The ability to drive from a desert to an area with heavy snowfall will now be possible.
Mt. Slippery is one of the highest peaks in the game. Mt. Chiliad will not be making a return.
One of the weapons is a remote-controlled explosive miniature truck called the "RC Tour".
There will be countrysides, deserts, and mountain peaks. San Fierro and Las Venturas will not be making a return.
The northern side of the map is bordered by the Tetas Grandes Mountain Range (the real-life equivalent being the Los Padres National Forest).
Children of the protagonist will be present in V, however they cannot be killed. It is unknown if they will be making appearances outside of cutscenes.
The protagonist has two children, boy Daniel and girl Skyler. They are about 10 years old and are too young to understand the protagonist's criminal history.
The protagonist is divorced, with the wife having full custody over the children.
An achievement called "Desert Bus" will be featured.
The hidden packages will be 100 crazy Sasquatches that will be placed around San Andreas. They will be recognizable by their faint red glow and constant cooing.
Attempting to boat around the mountains will result in a never ending row of mountains.
The eastern border of the map is the Yucca Desert, which stretches on into infinity. A single road runs through it, however traveling too far out will result in a message implying there's nothing out there.
The west and south parts of the map is the Pacific ocean.
The southeast part of the map is the Mexican-American border, however it is only referred to as "The Border". Crossing over the boarder will result in 5 stars, unless the player turns around immediately. Beyond the border is endless desert.
Dogs will be in the game, but only with their humans.
The story focuses on Mexican drug cartels, mainly between two warring cartels, X1 and the Cholula Cartel and their "army", Los Illuminados.
African-American gangs will return as the "Families Alliance" or even just "The Alliance". Their color is Black and Green. Their rivals are the Mafioso Alliance, who wear Blue. A third, smaller African-American gang called "The Tracers" will play the most important role in the story.
A Korean mob, Yakuza, and references to the Irish mob will be present, along with the Triads and Russian mob.
The protagonist's name is Ray, age 53, father of 2 children, divorcee, and an ex-con from upstate Liberty. Ray moved to Los Santos recently and was served with a divorce weeks later. Due to his criminal background, this prevented him from having custody of his children, thus giving full custody to his ex-wife instead.