I've never played any in the series, but from what I saw in Let's Plays of MGS: Ground Zeroes, it's just your regular stealth action game.
Why are people nerdgasming about this franchise?
Does it have something special about it? I mean, it obviously has to, to have such a fanbase, but I have no idea what so far.
The storytelling is incredibly complex and intricate, it touches on a lot of interesting subjects, as it discusses philosophy, nuclear war, the information age we live in, asking questions like "How much is a human being defined by its genes?" where in the sequel it asks "How much is a human being defined by information/by history?", in regards to the overall themes that drives each entry of the game. They are all tied together in that they each carry a similar message about 'passing information, genes, things that define you, on to the next generation' while delving into discussions on nuclear war with a vaguely anti-war outlook.
They all carry a seemingly implicit parallels to Nietzsche's philosophy, here and there, specifically "The Eternal Return of the Same".
Looking passed the engaging story, the gameplay is excellent and it's the small details that strengthens it; You can find so many details, both in optional codex conversations about the story, or all the gadgets you have at your exposal to infiltrate enemy bases can be used in various ways. Sometimes you'll discover a gadget's use which you probably didn't use before, sometimes you'll find new items that can utilized in certain situations to escape them.
Metal Gear is unique on that whole front.
It has changed alot from game to game with MGS3 beeing the best of them gameplay wise.
Story, yes -- Gameplay? Not so much. The stealth is complex and well done but the overall controls, aiming, are definitely not the best. That awards goes to Peace Walker.
All in all, I'm not as big on the franchise as I used to be before Guns of the Patriots, but the Phantom Pain is starting to resemble Peace Walker in terms of gameplay which has dragged my interests back. The only other games (among the actual newly released ones I've played) I could see as contenders for my favorites of the year would probably be either that one, Bloodborne, Life is Strange or Her Story -- I've mostly been playing older games as always.