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.