-Graphics are awful. Well, relatively awful. The lighting looks great, but one thing which really drags the graphics down is an excessive amount of blur. I'm not sure if it was upscaled from a lower resolution, or if it's a horrible attempt at a depth of field effect, but the blurry graphics really made it look like a mess. To make it even worse, it has the same problem as GTA:SA has, the framerate really really struggles in the game.
Compared to games like Assassin's Creed, GTA4 is just pathetic in the graphics department. Heck, I think even Saints Row looks better... Saints Row also struggled with the framerate but at least the graphics looks nice and clean. I just really really dislike in the blur in GTA4 but I suppose I'll get used to it. PC gamers should definitely look forward to GTA4 whenever it's announced because the PC version will most definitely be the best version graphics-wise.
-The overall feel of the game reminds me a lot of the original GTA3, only with a ton of misc improvements. The car handling feels MUCH better, it has a more realistic feel which I think works quite well. It reminds me a bit of the handling in Mafia. Close combat is also greatly improved where it feels kinda like a mini-fighting game (Face buttons turns into 2 punches, 1 kick and 1 evade). Gunplay is definitely a lot better although the controls takes some time to get used to (holding down aim makes the character autoaim at a target and you can do small changes to the aim (like targetting his head), or you can hold the aim button half-down to get completely free aim)
And the character's animations looks a LOT better. And he can climb over most objects with ease. Feels kinda like a mini-version of Assassin's Creed in that regard.
-Unfortunately, I couldn't find a single side mission in the game. It may be something you have to unlock, but I'm not sure. GTA has always had a great potential for side-missions so I'm hoping they are in the game and greatly improved. Saints Row did a good job of showing what kind of fun side missions one can add to a sandbox game.
-Police are much much MUCH better. As long as you're in the line of sight they'll behave similar to as in previous games, they'll simply send more and more units to your location. But once you lose their line of sight, they'll patrol the overall area close to where you were last seen. And if you manage to evade that area you lose your wanted status. I kinda wish they would have expanded the system even further and used the reinforcements system as NFS:MW so there was a point in actually stopping and killing all nearby cops to give a chance to escape. But still, the new police system is much more fun than previous GTA games. I randomly did crimes just so I could get a chance to do random police chases.
-Even though I saw no side missions I got a chance to try a few minigames. There's a simple bowling game which is a nice addition... but something you'll get bored of before you actually complete one bowling game. There's an arcade game which kinda plays like a variation of Tetris. Be ware though, if you're good at that kind of puzzle game you can easily spend a ton of time in just one session. My friend wasn't too happy when I spent more than 15 minutes on one game and I had to force myself to get game over.
-You can snap into cover Gears of War-style. I personally wish they would have used an automatic system like Kane & Lynch or GRAW, but it's something which works okay once you get used to the controls. From my playtime I didn't find much use for it and spent most time just shooting while crouching. It's really noticable on the enemies though, it's neat to see cops take cover behind their cars.
-Not very surprising, but as most console games the amount of options is very limited. You can't turn off the annoying blur, and there's only 2 control schemes to choose from (can't manually edit controls).
-You have several camera angles you can choose from while on foot or driving. One of the angles you can use while on foot is a camera which is basically identical to the camera from Resident Evil 4, which I thought was quite neat. And while driving you can make the game automatically use cinematic camera angles (or during missions make the camera lock on a target vehicle) and then you can make the game run in slow motion. That should be handy for those who wanna try making fancy videos with the game.
-Some people may like or hate this, but the camera system is very similar to previous GTA games. While moving on foot it's basically impossible to manually control it as it'll constantly rotate by its own. While driving the camera is really low and your own car will occasionally block your view.
-Button mashing sprinting. Bleh.
-You lose about 10% of your cash whenever you die, and you lose all your weapons when getting busted.
-You can instantly retry missions after failing them (I didn't see any function to replay missions after completing them though)
-After an early story mission you unlock the ability to call a taxi with your cellphone (although at any time, you can also whistle for taxis while moving around, but that's a bit of a hassle since there's not always taxis close to you), and when in a taxi you can quickly travel to any part of the city. Very very very handy feature.
-All of those stupid character stats from GTA:SA seems to be gone (thank god), but you do have kinda an RPG element with your friends in the game. All of them will have a respect and friend stat related to you which will go up or down depending on what you do. For instance, don't expect your girlfiend to like you more if you destroy her car while on a date. Or shooting cops while on a date. Or going to a strip club and beating people up while on date. (Gee, I'm glad I have GTA4 to teach me these lessons so it doesn't happen in real life.) And your friends may occasionally call you sometimes to ask for small favours or to just hang out.
-The first car I accidentally destroyed happened in an awesome way. I was driving really bad and crashing into just about everything, and then I started to notice a small fire from the engine (in true GTA-fashion that's a sure sign it's gonna blow up) so I quickly stop the car and run out. Unfortunately, I didn't notice I stopped the car near a gas station. Big bada boom.
-Speaking of which, there's a few other new features related to cars. Niko is a lot slower than other GTA heroes when breaking into cars (if you see a car which is locked, you might wanna quickly shoot the window to make it faster to enter the vehicle). Sometimes you won't get a car to explode after driving like a maniac, the car's engine might simply stop working instead. And cars which are upside down no longer have the spontaneously-explode syndrome.
-Similar to as in Saints Row you can jump out of a car while it's moving. But do NOT expect it to be as safe as in that game. Falling and getting thrown around can hurt a lot, and looks quite painful thanks to the euphoria system. In one case I died in a very spectacular fashion. I was driving a big truck (well, I can't remember specifically what I was driving, it was big anyway) and the car started to spin out of control so I decided to simply jump out of it. While jumping out and landing on the ground, the car spins so much out of control it falls on its side. It fell on its side *right* on top of me. Which hurt quite a bit, and made me look like the worst stuntman ever,
-To make it easier to navigate you can target any part of the map for your GPS. And then you'll see the quickest route to that spot on your minimap. You can even turn on speech for the GPS so you can occasionally hear how long is it to the destination and when you have to turn.
-Radio channels are awful. There's a ton of rap and hip hop which I personally can't stand at all. I did find one radio channel called The Journey which had a lot of music by Jarre which I found enjoyable. Unfortunately, I saw no function to use a custom soundtracks as a radio channel.
-Typical GTA humour is definitely in the game. So people who like bad jokes with too much sexual innuendo will feel right at home.
Well, and that's all the stuff I can remember. Overall, it definitely seems like a good game and I'm really looking forward to try multiplayer. But I have to say I'm a bit disappointed in a few areas, like the graphics and how the overall gameplay hasn't changed too much since previous games (although specific gameplay elements like police, car handling, aiming and such are vastly improved).