I'm never going to understand MGS series fans, hahaha.
Yeah, MGS breaks people into a lot of different groups. There's people that play MGS for the story, people that play it for the gameplay, people into the lore, people who hate the cutscenes, people who dislike the gameplay.
For example, my brother's an MGS fan. He plays all the console games since MGS1. He didn't think MGS3 was anything special because he just ran through it once to see the story and then was done. Each area was just a linear experience of getting to the next point to see the next cutscene. For him, GZ's would be lame because he'd run through it in an hour for the story and then never touch it again. He liked MGS4 because it was just another linear MGS4 story experience, no different from the rest but bigger explosions and better graphics.
Otoh, for me, MGS3 is my favorite game of all time. When I play MGS3, each area I think "can I do this?" with dozens of possibilities for getting across the area and just doing things because I can and then when I try them and most of them end up being possible it's pure fun. The game is just a giant playground with neat swings and slides and toys to play with. I love finding all the secret codec calls and hearing medic joke with Snake about godzilla movies and Major Zero's James Bond fanboy-ism creep into his work. I like finding the little frogs and throwing snakes at guards because...because I can throw snakes at guards! It's just such a kid fantasy game of having a blast along with flying saucer guards, ghost valleys, bosses that can die because you turned your system off for a week, epic chases, and heart-felt moments. It's really the perfect game for my tastes, fantastic replayable gameplay and a story with lots to get out of it and a very solid main tale. But if you just run through the story in 12 hours from start to finish, you don't get enough half of that experience.
MGS4 started open but ended up being a linear movie experience, and a shitty movie at that trying to die all the nonsense lore together. GZ's is taking the opposite route and going back to MGS3's "here's the key's, now go do anything you want". Even for the story being darker in GZ's there's still tons of jokes, in-joke easter eggs, silly ways to fuck with guards; the campy charm is still there. But like MGS3, GZ is a game where you gotta want to have fun to do so. PP will probably be the same if GZ's is a good indicator. It's basically MGS3-2 with every aspect (outside of plot) being taken with a generational leap forward due to new ideas and just simply new possibilities of having a giant integrated sandbox world.
My original worry about MGS5 being sandbox was that I've never played an open-world sandbox stealth game that was able to keep the intensity of carefully designed levels that test stealth skills. Also most sandbox games are all too easy and there's little challenge (see Assassin's Creed). Thankfully GZ pulled it off wonderfully. The game is challenging and guards, cameras, noise, darkness/light, explosions are all always a constant threat. In the open areas, just by changing the guard spawn locations in the different missions, you get incredibly different experiences and you're always on alert and playing the smartest you've played in a stealth game. Most realistic as well. And then inside buildings the game transitions perfectly into MGS1 style level design set pieces with guards at specific spots and you needing to find your way through the levels with options, but fewer options.
Yeah, I know I'm an MGS fan and I gush a lot when MGS is good, but that's because when MGS is good, it's the highest point of the game medium to me, personally. I hardly ever replay games, even my favorites. But MGS1-3 I've played 3-5 times each easily, even Portable Ops and Peace Walker I've played at least twice. It's just such a fun series to play. MGS4 burned me bad, so bad that I wanted nothing to do with the series anymore, and was embarrassed to be an MGS fan because it was such awful fanboy drivel. For the last 6 years I'd given up my MGS fandom, and didn't even watch the announce trailer for Ground Zeroes as it was just "meh, more crappy post-MGS4 mgs", but after playing it a ton, I was totally wrong and GZ is the real deal and the true sequel to my favorite game of all time, so yeah I'm a little excited since it's been 10+ years since I've played an MGS game this good.