To actually answer the question:

Consistent novelty.

Metal Gear on MSX2 was a genre that didn't exist yet, and which sounded boring when he proposed it. The real Metal Gear 2 built on that in ways that would still be noteworthy features in Metal Gear Solid. Snatcher and Policenauts were little more than visual novels, but they were well-executed. That's largely thanks to Kojima insisting on artist-driven tools for scripting the exact timing of graphics, text, and music.

Metal Gear Solid fucked with the player by constantly breaking the fourth wall. MGS2 cranked that ten times higher, along with prescient comments on memetics and populist narratives. MGS3 was just polished as hell. MGS4 opens with fake commercials starring the voice actors and only gets weirder from there.

MGS5-- calling back to artist-driven tools, I recommend the article about the game's rendering engine. They developed a little rectangle you can drop into a screenshot, and then however you adjust the screenshot in Photoshop, copy-pasting that little rectangle back into the game will perfectly match whatever you did. Kojima productions have a certain "just solve the problem" vibe behind a lot of their technical direction. MGS 1-3 had too much focus on the minimap radar, so MGS4 has a holographic ring around your feet. Why? How? Who gives a shit, it's a video game.

P.T. was a horror game demo set entirely in one hallway. And it was terrifying. And weird. And full of promise. So when Kojima handed that gift to Konami, reviving one of their beloved franchises, with several big names on-board thanks to his weird industry connections... and then Konami booted his ass out the door... people noticed.

Death Stranding is the ultimate illustration of why he became well-known and why he remains well-known. It's a ridiculous product. It forces comically long sequences that are not technically gameplay. Its writing is completely bonkers and longwinded. But every aspect is deliberate. It is that way, on purpose. A premise that sounds boring becomes interesting because it's well-executed. Balance and stamina aren't floating UI elements; they're represented in your character's movement, so you keep your eyes on your dude.

Basically, Kojima is the sort of lead who can insist on a ten-minute opening cutscene, thirty seconds of actual gameplay, and then another eight minutes of cutscene, and still have people's attention.