Seriously though, people keep saying story was bad but the gameplay was good and that we can divide it into those camps. But I'd divide it into 3:
Story = Bad (but who cares? Bayonetta has a bad story and I love it)
Gameplay = The mechanics were great and what most people love about the game. Snake feels great, his weapons are cool, and there's a lot of ways to approach each mission. It's a fun little sandbox, but my god does it show its limits when we get to the structure of the game, which is my next part:
Structure = This personally my biggest issue with the game. It's a mess. From repetitive missions (both main and ops), a really badly laid out open world that's pretty much just challenge maps with space between them, the really egregious real time timers on weapons, The constant intros and outros to missions, having to leave mother base and come back to initiate some scenes, the quarantine missions being poorly set up, and the fact that the game's structure (NOT narrative) has no gameplay payoff, are all just way too much for me to ignore. Upon arriving in chapter 2, it became immediately apparent that this game wasn't finished, and I'm not talking about it from a story standpoint. The game loves to waste your time.
I think these all have different weights for different people. Some can ignore story and structure to revel in the sandbox nature of snake's abilities. Some, like me, can ignore the story, love the gameplay, but have significant issues with the structure's inability to remain interesting. I think people can be pulled by any one of these three factors. I think you can have a game with mediocre gameplay mechanics, but a highly interesting structure that builds as you go. I think you can have a game with a great story, but not so great gameplay and structure, and have that be enough to draw in a specific crowd.
It's also important to note that I can bring up and talk about all of these elements without comparing it to past metal gear games. I judge a game on its own strengths, not what the rest of the series is.