Here's a list from WikiGTA of known things going into the game, and there are more that I haven't listed on another site, but I'll see if I can dig them up later:
*
A highway is included in Liberty City, with working toll houses.
* Shooting a car will result in realistic bullet holes. * Tires can be popped.
*
The map is smaller than that of GTA San Andreas, but no room will be wasted for large empty spaces like deserts or country side.* All streets have real names, and all locations are at real addresses. - Holy crap, considering how big the game is.
* Liberty City is based on four of the five boroughs in New York and an area of New Jersey. Liberty City is the only city in GTA IV.
* The game starts with Niko helping Roman, who is heavily in debt.
*
Cars can be stolen by smashing a window to gain entrance.
* You can use a mobile phone to call others, and not just to receive calls. The phone may have some other functions, but no details are known. * You cannot fly planes in GTA IV, but you can fly helicopters. A Maverick-like helicopter and a combat helicopter are available. +Liberty City has one airport. - THIS ONE PISSED ME OFF, but whatever...
*
Niko can climb into telephone poles and fire-escapes. * You can change Niko's clothes, but that's where the customization stops. Don't expect barbers or tattoo-shops.
* The mafia makes an appearance, but the mob won't have an important role.
* There will be 'virtually' no characters from previous GTA's.
* It will be possible to have girl friends.
* The camera is positioned closer to the car and lower to the ground than we are used to, and can be compared with the camera in Midnight Club.
*
GTA IV won't need loading, after the initial loading screen. * The length of GTA IV can be compared to those of previous GTA's.
* The Xbox 360- and PS3-version of GTA IV will be more or less identical, although exclusive downloadable content will be available for the Xbox 360-version.
* GTA IV does have multiplayer, but this won't be a massive feature. Further details are unknown
*
Pedestrians and traffic will behave according to the time.
* GTA IV is, unlike previous GTA's, not inspired by certain movies.
*When you enter a building, the outside world can be seen through windows, doors, etc., and will still be moving in real-time. - Woah. ...Whoa.
* GTA IV will have more features while you're not playing a mission. * Boats are available.
* Philip Glass, who wrote the trailer music, is on the soundtrack development team.
*
It is possible to open the trunk of your car, probably to store and transport items in it.
