If you put the two side-by-side, the clearest difference I notice is intent. Mainstream anime is often engineered to catch wide attention — energetic openings, distinct archetypes, episodic momentum, and clear emotional payoffs that feed fandom and merch. Adult-targeted anime aims for depth and nuance: ambiguous protagonists, slower reveals, heavier themes, and sometimes graphic content. You’ll find adult stories in seinen and
josei manga adaptations, late-night TV slots, or festival films, while mainstream titles usually dominate TV primetime, kids’ blocks, or multi-season streaming pickups.
I also feel distribution and tone play a role. Adult works might surface on niche streaming blocks, in theaters, or have limited runs because they're riskier commercially. Creators get more freedom to explore darker politics, sexuality, or existential horror when the audience is explicitly adults, so the emotional palette is different. For me, both have their joys — mainstream gives the rush and community, adult anime gives the kind of brain-tickling payoff I replay in my head for weeks.