MMORPGs
No ending makes it so the game, for me, lasts and lasts. I've been playing WoW for 6 years, because there's no end to the content, experiences. I played through countless RPGs, and once I was through with the story became bored, and moved on. Though I kind of wish I could entertain myself, like my younger brother does, with Fallout3 and Metro 2033 on countless play throughs.