There may be some spoilers in this post, but only very minor ones that happen early in the game. I'm not going to spoil any big pieces of the plot, so don't worry about anything being ruined for you.
First thing is first. When compared to older GTA games, I believe that this one is the best. Also, if you're a fan of FPS games or third person shooting games, I seriously don't understand how you can't enjoy a GTA game. I know a lot of people on these forums mention how they don't like GTA, but they play games like Halo or Gears of War. If you like to shoot people and you like action, then you shouldn't have any problem enjoying GTA IV. You may not like it as much other people do, but to completely dislike the game seems a little far fetched if you are into the shooting genre of games.
I'll start off with some positive things about GTA IV. Everything has been revamped and made better. The aiming system is 10x better, the physics are awesome, the AI is far more intelligent, and the cars handle much better (although some people beg to differ). Shooting people dead in the old GTA games wasn't near as rewarding as it is now. Blood effects on cars great, as are the cracked windows or body damage. Shooting a guy in the head while he is laying on the ground dying causes blood to pour out. Also, many times you will shoot people and they will squirm around on the ground, half alive, all because of the new physics engine. Shoot people in the leg and they limp. It all makes for more gruesome fighting. This isn't to say it's a perfect system, but compared to old GTA games it's godly, and compared to any modern FPS it's still good. The cover system is nice, but not as advanced as games like Rainbow six. Even though it acts glitchy sometimes, it works well with the new aiming system, and it's better to have than not.
The story is entertaining, although a bit cliche. It's not much different than any Martin Scorsese film you've seen that uses common gangster stereotypes. Personally, I find the best thing about the story to be the characters involved in it. They are the best GTA characters yet, and Niko is by far the best protagonist, and generally one of the most interesting characters I've played as. The guy is just downright likable. He is witty, sarcastic, and humorous.
The city is great. Compared to other GTA games, this is my favorite city. If you have an ice TV, the city just looks plain sexy. It's not as big as San Andreas, but I think that's a good thing. Driving around in SA got extremely boring. From one city to another, cross country, felt more like a waste of time than fun. It was cool at first, but the novelty wore off. As for Liberty City, I always enjoyed driving around. It didn't take too terribly long to go from one place to another, and the city was more lively. You can also whistle taxis down, so if you don't want to drive to the other end of the map you don't have to. The game also did a great job on building exteriors, it can be hard to tell which buildings you can and cannot enter at first. After a while you learn what buildings are usually accessible, and you can always tell by the building's door whether or not you can enter, but when you first start playing it can be hard to tell. They did a good job with the little details.
The missions in the game are fun, but can be sometimes repetitive. It's not much different than old GTAs in that aspect. There isn't a huge variety in missions. You have chase missions, warehouses full of people that need to die, single target missions, rinse and repeat. Honestly though, they are still fun to play, even if you do end up doing one that feels very similar to one you've already done, and I think this has to do with the new shooting system. To be honest, I don't find it any less repetitive than a game like Halo or Call of Duty. You will always find patterns in these types of games.
Now I'll discuss some things that were disappointing or not as good as they could have been.
As you know, they took out a lot of things. Helicopters, parachuting, bicycles, and that all sucks. One thing they took out that really bums me out is customizing cars. Infact, throughout the game you don't even get any car garages. You get safehouses, and outside of them are spots on the street to park a car, but it's nothing like a car garage from old GTA games. It seems like they took some emphasis off vehicles. You still drive them just as much, but the whole vehicle collecting of old GTA games is pretty much obsolete.
There is a new feature they added to the game that seems cool at first, but you slow realize it isn't such a big addition, and that is the ability to make choices. Very early in the game you get a mission to let a guy live or die, a few missions later you get another one where instead of just killing a target you execute him and it goes into this little cutscene. They introduce these two new concepts to the game that seem interesting, but as time goes on you soon find out they are rarely used. After playing through the entire game, you are only given the option to let someone live or die on four different occasions. Just four, out of a truck load of missions. Sometimes you think you'll have that option during a mission, and then you find out you don't. These things don't really make the game better or worse, but you wonder why it was even added to the game for how little it's used. There are some other choices you make in the game that a bit different than letting someone live or die, and they are much bigger parts of the story. I won't go into those, but I will say they were done much better than the live or die options.
Another disappointment is multiplayer. A lot of reviews have talked up multiplayer and how great it is, but I gotta admit, it's pretty stale and watered down. The free roam is enjoyable, because it's exactly what it sounds like. Just run around the city with friends doing whatever. The only downfall in that is you can't play mini games like bowling or pool, other than not having any mini games, free roam is a fun addition. The more competitive modes, on the other hand, are a let down. I'm not saying things like Team Deathmatch aren't fun, they just aren't a good alternative to more competitive games like Halo or CoD. Sometimes the areas seem too big and it's hard to find people, and when you do you die and have to respawn and go around collecting guns again. Instead of a chaotic atmosphere like you would expect, it's a lot slower paced. It can still be fun to get in a car with a buddy and chase another car down shooting at them, but if you want to play strategic, skillful and competitive shooting games, stick with CoD. I think that for online team death match game modes, Saints Row did a better job than GTA. But being able to free roam on GTA is awesome.
More could be said, but I am done. If you're not a fan of old GTA games, you should still give this one a try. Rent it and see what you think. Don't let the over hype kill the game for you. You can't expect gold, just be content with silver. I loved it, personally, even with the stuff that disappointed me. Simply put, the game is fun. The story mode was fun to play through and I plan on playing more still to dick around in the game, do some more side missions, and play online more.