I'm going to say something I never I would say, but Dorotea is right: Joss didn't wholesale rip off Lost.

The Harvest

Giles: This is what we know. Some sixty years ago, a very old, very powerful vampire came to this shore, not just to feed.

Buffy: He came 'cause this town's a mystical who's it.

Giles: Yes. The Spanish who first settled here called it 'Boca del Infierno'. Roughly translated, 'Hellmouth'. It's a sort of, um, portal between this reality and the next. This vampire hopes to open it.

Buffy: Bring the demons back.

Xander: End of the world.

Willow: But he blew it! Or, I mean, there was an earthquake that swallowed half the town, and him, too.

Giles: You see, opening dimensional portals is a tricky business. Odds are he got himself stuck, rather like a, uh, cork in a bottle.

Xander: And this Harvest thing is to get him out.

So yeah, Lost doesn't own that metaphor. What's more, Joss used it thirteen years ago to again reference the Master, the Hellmouth and the mystical convergence thereof.

So... yeah. It's kinda like accusing Joss of ripping of Twilight by writing the Buffy/Angel storyline in Season 2.