Partly the nanomachines, partly Kojima's betrayal of what he started in MGS2. Having 'Liquid' turn out to be Revolver all along, explaining Vamp through nanomachines, having Raiden become a cyborg ninja for no reason other than the fact that stupid people didn't understand his character in MGS2 and not explaining in-game how he became said ninja. Don't get me wrong, it was cool and I liked seeing him take down Geckos and fight Vamp as much as anyone, but the execution sucked and it contravened his character development in MGS2. It obviously wasn't the direction Kojima intended for the character when he made MGS2. As someone whose first Metal Gear game was MGS2, I had an attachment to Raiden's story and felt it wasn't handled well in MGS4. Drebin did my head in, especially his fifteen minute monologues every time you beat one of the Beasts (who were poorly explained and rubbish themselves, although they had fun boss fights.) Sunny was created for literally no reason other than to tell a guy who's going to die in a few months smoking's bad for his health (seriously?) Then the ending. If it had ended with Snake offing himself, I'd have been fine with it. But then... Big Boss. I love the man as much as anyone, but he completely ruined MGS4's ending for me. It was a story about Solid Snake's end, and was handled beautifully until that point. Then Big Boss completely stole the show, with Snake hardly saying a word in the end as he monologued for nearly twenty minutes. He even got the last line (not counting the phone call at the very end), before dying in such a lame way - the man survived death, and finally gets topped by a plot point that should have been left after MGS1. I liked his and Snake's reconciliation a little, but it was kind of forced and it was hard to see the man who's effectively insane in MG1 and MG2 in the Big Boss of MGS4. I didn't like it. Snake needs another game to properly end his tale, because his conclusion in MGS4 was overshadowed by Big Boss just because he was a fan favourite after MGS3. In terms of gameplay, it's got the best in the series but there's so little of it. Real stealth goes out of the window after the first two acts - act three was just plain awful, act four had the potential to be amazing (return to Shadow Moses, dammit!) but was ruined by a lack of human enemies in the base. The REX fight kind of redeemed it. Act five was pretty dull and mostly cutscenes, though the Shrieking Mantis fight was cool and the Liquid Ocelot fight was genius. But overall, there wasn't much stealth gameplay. Or gameplay at all, for that matter. It was a good game, but didn't hold a candle to MGS3. Hopefully Ground Zeroes is more a blend of that and the meta-gaming of Peace Walker.

For me it was timing, I played MGS1, 2, and 3 around the time they came out so everything was fresh and innovative. I played MGS4 about three years after it was released.

Story was garbage, the cutscenes happened every 15 minutes. I found the whole game annoying and frustrating, just wanted to sit and play.

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.

One of the reasons is that it's essentially a $60 movie.

WHAT. I LOVED MGS4. One of the best games ever made.

i dont hate it...

Also, I think people got annoyed of the lengthy, frequent cutscenes.