I think what is most depressing about MGS4 is the dip in quality the game takes after Act 2. The game is pretty amazing up until then, the story hasn't shit the bed yet, and the gameplay, sneaking through actual battlefields, was a great way to shake up the formula of previous games. The enhanced controls were also awesome. What gets me though is that the game peaks so very early.
Act 3, is a mess, though it didn't bother me that much at the time, it was Noir, which I am a sucker for, and the city, while lifeless, had great atmosphere. The end of the act though was the beginning of me actually getting really, really bored with the cutscenes for the first time ever in an MGS. The scene with Ocelot taking over the soldiers could have happened in about 1/3 the time Kojima took and still been effective. The revelations prior to that seemed cool at the time, but don't stand up to much scrutiny.
Then we get act 4, the return to Shadow Moses should have been great, but it's all dodging robots. Dodging robots is boring. Very boring. The tools you have are limited, and the stealth there just didn't feel great. Boss fights weren't bad though.
Act 5 was essentially one long cutscene, with the best part being the microwave hallway. The stealth section at the beginning, also far worse than Chapters 1 and 2.
As far a depressing story, seeing Snake slowly die is painful, but most of the other emotional scenes didn't work for me. The deaths of Eva and Naomi in particular were supposed to be dramatic but ended up being comical, especially in hindsight. There was no reason for them other than melodrama, and it showed.
