For me, the beauty of Glorantha is in its depth, consistency and, to a certain extent, its mythic nature.
It is deep for several reasons. A lot of the game world has been described, at least partially. Many regions have been described in depth, both in official and fan publications. The backgorund covers nearly 2000 years if history and a huge amount of other, pre-Time history. Each culture has a store of history, myths and legends and the interactions between them are fascinating.
Glorantha is generally consistent in that not a lot is retconned out of existence. Having said that, Gregging describes when an idea has been changed to make the original idea no longer valid, so things do change. However, to a large extent, what was written 30 years ago still has a lot of validity.
Glorantha is mythic in nature because it has a Pre-Time history of mythology and legend that has effects on the world. Gods, Demigods, Demons and Heroes were real, existed and can generally be contacted. Normal people can take the Hero's Journey and become Heroes themselves. Gods are personal, to an extent, and have their own natures, personalities and aims. Worshipping a god or goddess not only defines you as a person, it tends to indicate what career you follow, how you live, what you can do and how you act. Memebrship of a cult brings a new richness of flavour that is lacking in many games and settings.
Personally, I'm not that keen on some of the touchy feely Glorantha - myths affecting every level of life, people acting ritually all the time and that kind of thing. It's fine but should be kept at a distance, in my opinion.
The technology is said to be Bronze or Iron Age, but that is really misleading. The common metal is Bronze and Iron is a very special magical material, but so what? You still have metal weapons that do damage, it doesn't really matter that these are bronze not iron. Praxian nomads move around the Wastes using some metal but not a lot; Sartarite hillsmen live in small clans and family groups and use metal, herd cattle and raid each other; Western townsmen live in cities, worship in cathedrals and have a higher level of civilisation. But, what has this got to do with Stone/Bronze/Iron Age technology? Not a jot. Dwarves have steam technology and firearms and Malkioni have flying ships, so what level of technology is that?
Above all, Glorantha is what you make it.