In the case of MGS4 and MGS5, because they're weirdly horny... Like, MGS5 was a good game, but you can't tell me that Kojima doesn't spend a lot of time thinking about bikini clad snipers.
I haven't played 4 (couldn't get past the cutscenes), but ah... Quiet is definitely front and centre in 5 at a certain point in the storyline, very embarrassingly so.
I was top 1000 for a while in the base-invasion mode of MGSV in the EU. That mode was basically a low-budget little tacked-on multiplayer that kept the core-gameplay and I spent hundreds of hours on it.
MGSV Metal Gear Online (the _other_ tacked-on multiplayer) was also great, but very unreliable netcode made it frustrating.
I wish someone would just take that game engine and make another good game on it. I _almost_ bought metal gear survive on the engine alone, that is how good it is.
I think the other trademark is Kojima, when he's not being weirdly horny, is just "supernatural and weird". Like the other stuff with the floating boy/psycho mantis, man on fire, parasite unit. When you've got the weirdly horny and supernatural mashed together, it's either a Kojima game or some variant of Biohazard (Resident Evil).
My overriding thought would be, ridiculously convoluted story, which maybe-sorta kinda works by some insane internal logic. More like the author thought X would be cool, with no thought of downstream ramifications to a coherent plot.
If it fits in 6 minutes it's not so convoluted, there's a guy who made a (10 hour?) video trying to explain all the lore and story of Destiny 1 and Destiny 2 (and all their various DLC/paid seasons). And the guy who made this apparently lost interest towards the end of Destiny 2 as a live service game so there's easily another 2-3 hours of content covering the final beats of the story.
In the case of MGS4 and MGS5, because they're weirdly horny... Like, MGS5 was a good game, but you can't tell me that Kojima doesn't spend a lot of time thinking about bikini clad snipers.