This game is heavily story driven. Even so it is still the best game you could buy.
The controls have been changed from MGS3 to MGS4 but not in a bad way. R1 is now the trigger and it feels much better than the square button. A few other minor changes have also occurred. The only thing I do have a problem with is the lack of explanation when it comes to the controls. They leave it up to players to figure out for themselves things such as when in first person press the right or left on the digital analog to peek to the side.
The story continues some time after MGS2 and tries to close up every inch of the story and any unanswered questions. Well it does that and bloody more. It gives you a conclusion worth ten headaches. To remedy that the people at Konami decided to release MGS database which is an encyclopedia/timeline/relationship chart/ a lot of information.
The game it self is not perfect but it also gets a massive boost from MGO. In Metal Gear online you play as a soldier with the SOP system who fights with various different modes and some classic ones as well. MGO now has a system of skills to use which most consume 3 while the weaker ones consume 1 point at the maximum level which is fine but you only have 4 slots to work with.
All in all try out the others to get some what an idea of this game. Then if you like those you'll love this one.