I haven't played MGS4 myself, but after playing the three previous parts and watching the reviews, I think the main problem with this game being GOTY is how exclusive is it.
As in inclusive vs exclusive. The point being that it's extremely plot-driven, but understanding anything of the plot requirs playing the entire series first, and even then it gets pretty ridiculous. A lot of people also don't like the ridiculous amounts of cut scenes, and the gameplay is pretty hard to get into.
Definitely GOTY for MGS fans after fanservice, though. =P







