Basically, Glorantha is the fantasy world a brilliant Californian hippie shaman who hadn't read Tolkien would create. I think it's unrivalled among secondary worlds except by Middle-earth and Tekumel.
This isn't quite true. Glorantha predates Steve Perrin's RQ rules and its development was never closely tied to them. The (excellent) HeroQuest books contain almost no additional rules, and since HeroQuest stats are all in the form [Verb or Noun] Number, their material is easily usable in any game system.GrumpyOldMan said:Glorantha the game world was originally designed around RuneQuest the game and much of the early material has a lot of RQ rules. Since the publication of HeroQuest (which is completely incompatible with RQ) the game world material contains a lot of HQ rules.