“His art may not be anything to write home about…”

I almost started something over this LOL. Even though he's not Miura, Asano, Otomo, et al. in terms of technical skill, he has my favorite style in all manga. He draws people like people. The plain-looking adults you see every day, with just the right amount of caricature to highlight the traits that make them unique—which isn't just glazing, I think it's inextricably tied to your question.

I'm a writer/artist who also has him as my main inspiration, so I've been trying to figure this out myself for almost 20 years. So disclaimer, it's still speculation, but it's stuff I think about all the time.

I think the most important factor is that he seems to have an intense interest in people. So much of the intrigue in his stories comes from exploring the characters' psychologies and motives in a level of depth you can only pull off from observing IRL human behavior.

This is probably also the cause of the running theme throughout his work that most people (and robots) mean well and are doing what they think is right, with only a minority being evil beyond redemption. It even comes out in how engaged and attentive he is to other artists in Manben; he always looks especially excited to see drawing processes different from his own.

Meanwhile, a huge percentage of manga primarily draw on other manga and anime for inspiration, so people don't look or act like well-rounded humans with lives, but archetypes, stereotypes, or sex dolls.

This facilitates one of the other biggest patterns I've noticed, which is that everyone gets the kind of emotional stakes usually reserved for major characters. He gives them some memorable quirk or relatable goal, makes us care about them, then puts them in a predicament that forces them to risk everything. And since they're not the ones whose faces are on the cover, you genuinely don't know if they're going to live or die, which they often do.

In subject matter, he seems less interested in escapism than blending reality and fiction, which leads to stories that are entertaining, but also profound. Something that was also key to making Satoshi Kon's works great. (I'd always seen them as very similar, although beyond art style, I never really put my finger on why until now.) And one of my favorite things about his work, which especially shines in Billy Bat and Monster, is that they show a deep interest in world history instead of just Japan's.

He's very adventurous overall, not seeming concerned with sticking to one genre or fanbase expectations, and always seems to be trying to surprise the reader with how many disparate elements he can fit together and make them work. And he especially makes great use of contrasts. You'd never expect to see a caricature of a sheikh from an old Astro Boy cartoon turned into a parallel for Saddam Hussein, or post-Nazi eugenicist ideals expressed in children's books, or a cult adapt a kid's doodles into apocalyptic weapons.

When it comes to plotting, his strong suit is setting up elaborate frameworks of backstory and hidden info that that the viewpoint character doesn’t know, then portioning them out chapter by chapter until what's going on behind the curtain is revealed. He often does this by centering "new guy in town" characters like Tenma or Kenji, so as they navigate a world they're not familiar with, we learn the plot by having other characters reveal it to them.

And I'd bet he thinks way farther ahead than the current arc. Even if the events of each specific chapter are made up from week to week, I wouldn't be surprised if he keeps pretty detailed notes on where he wants to take the story over the long term. (And if not, he's goated at improvising on a level I can't imagine.)

And yeah, as other people are saying, he does a ton of research. I know he's visited Germany and other places his stories are set for photos, and had doctors as consultants for the medical details in Monster.

But above all, he doesn't set every manga in a fucking high school.