Okay, I'll be the slightly annoying pedant and say it's not that different at its core—it's still a 'one guy, many women' premise. Where it really diverges, at least for me, is the sheer scale and the premise's brutality. Most harem stories are set in a stable society where the guy just has an unusually magnetic personality. Here, a virus has literally killed 99.9% of males. The narrative isn't just about picking a girlfriend; it's about a single man's psychological and ethical responsibility to repopulate humanity while being hunted, manipulated, and idolized.
The power dynamics are flipped entirely. In a typical haren, the women hold a lot of social power; the guy is often the underdog. In 'World's End Harem', Reito is the ultimate resource, and the women—whether they're government agents, scientists, or other survivors—are constantly fighting over control of him. It takes the fantasy to an extreme, logical conclusion, which makes the ethical dilemmas way messier and often more interesting. The manga gets really dark exploring those implications, which a standard rom-com haren would never touch.
Still, gotta admit, a lot of chapters still fall back on very familiar ecchi tropes and power fantasies. The dark premise sometimes feels like a veneer for more of the same.