The gameplay is not so much changed as it is enhanced. I played a lot of MGS3, and the sneaking and such worked very similarly, only with the addition of some awesome new CQC moves, like smothering. The Metal Gear Solid series discusses the role of the soldier pretty heavily, and since soldiers don't just sneak in the shadows, MGS4 has made gun-play better than ever by adding an over-the-shoulder view, which is extremely useful.
The gadgets are tons of fun. The sheer volume of weapons is very refreshing from MGS3, which didn't offer too much variety when it came to that. Thanks to a man known as Drebin, the player can customize the numerous weapons throughout the game, all of which are a lot of fun.
The boss battles are as great as ever, though the camo you recieve for tranqing them out is a little bit different than how it was in Snake Eater. You get to have the face of your opponent now, rather than just their skin. Anyway, between the Beauty and Beast unit and all the others, the boss battles are just as good as the serie's reputation has them, if not better. I'll not talk about the specific boss battles, but I think that they got better as the game progressed, and my favorite B&B was Mantis.
In terms of graphics, this game is unmatched, and probably will be for a long time just as its predecessor was. Partly, it's a clear technical mastery of the medium, but also, the taste is outstanding. The levels simply look outstanding, not necessarily because of raw power but because it's so pleasing to the eye, and that should be the continued aim as graphics technology gets better. This game looks to capture the beauty of people and flowers and sunsets, all in a 'tactical espionage game', and it accomplishs this with wild success.
It's true, so much of this game is cinematics. It aspires to be a cinematic game. The gameplay is in itself great, but it's within the context of a story. I don't know why developers haven't tried with this concept before. Authors use the structure of their sentences to try and echoe their larger point that the contents of their novels convey; why shouldn't video games work the same? This may be the last game of the series, and it ties up everything it ever could with that being the case (watch all the way through the credits). The vioce acting is great, and everything looks terrific. I don't want to say this game is quite philosophical, but it certainly aspires to some kind of philosophy, and it seems to merge it a bit with some kind of comment on the structure of society, which is a difficult thing to do.
Over all, I'd say this was worth about $550, including shipping.