Nanomachines seems to be the most common complaint, along with there being too many cutscenes. I really like it, but it is probably my least favourite main game in the series (Except Portable Ops/Twin Snakes). I find the concepts of the story, like the war economy interesting, but the story itself... not as much. Metal Gear Solid 4 has less interesting villains than the rest of the series. Liquid Ocelot is downright silly at times, especially during the scene where he controls SOP. Liquid Snake, Solidus, The Boss and even Gene were more interesting as antagonists in my opinion. (However, the final battle with Liquid Ocelot on top of Outer Haven is arguably the best final battle in the series). The B&B corps are terrible. They have over-the-top backstories that are impossible to take seriously and the depth of their character doesn't go beyond their names. Hell, Raging Raven even shouts "RAAAAAAAAAAAAAAAAAAGE" constantly. Finally, Vamp fell victim to nanomachines, along with a lot of the story. I have mixed feelings on the changes made to Raiden, but his parts were well done (well, except having a dramatic sacrifice with him remembering his words to Rose, only to be slightly wounded and return almost immediately), so whatever. As for the gameplay, the mechanics were improved, but the level design went against it at times. The first two acts were fine, though the setting wasn't very interesting in either (and yeah, I know it's a war-zone). After that, the game not only got shorter, but worse. Act 3 was bad. It had a great setting, but the fact that you are forced to follow the resistance guy through the entire thing punishes exploring and taking things at your own pace. The enemy placement is also bad, since most of the guards stand in one place the entire time. I don't know what the thought process behind Act 4 was. There are no guards during the mission, only those stupid scarab things, which are the most annoying thing in the series if they see you. Act 4 makes up for it at least slightly with great boss fights. Act 5 was as good as the first two acts. The problem is, it's ridiculously short. Anyway, MGS4 is still great despite all that. The ending is amazing, and I really liked how they concluded most of the character arcs. One of my biggest problems with the game, (installing every act over and over) was actually fixed.