I think what makes this game terrible is the way the story and gameplay clash. The game tells a pretty damn grim story, much like ground zeroes. I mean, that game played in the Guantanamo Bay detention centre and didn't whitewash pretty much any of it. And this game follows suit, the story is one of betrayal and war atrocities, and with almost no levity to it. It's all child soldiers, dying children, pandemics, corpses... But that's the thing. The gameplay is as MGS has always been, downright goofy. You can glue recruitment posters on your cardboard box and fool guards with it. You can glue pinups on your cardboard box and guards will come to ogle at it and drop their guns. You can Fulton guards out of base so you don't have to hide the unconscious bodies. You can call in a supply drop over an enemy sniper and knock him out with the falling, green plastic box. You can command your horse to defecate. In short, you can do tons of really silly stuff in this game. And then you go back to the missions and it's a bunch of gravelly voices talking about war and child soldiers and bombs hidden "where noone would look" (that's code for vagina apparently).
Hell, you don't even get silly bosses anymore. Everything is so damn grim. MGS 4 had a grim story, too. But at least the boss fights were suitably silly in setup and design. MGS5 doesn't have any of that.