What's so special about Glorantha? That is a very tricky question to answer.
Imagine a world where the Ancients were right. Where the earth was flat, and you could fall off the edge. Where the sky was a solid dome, with holes where the light beyond shone through - the stars. Where the sun rode across the sky in his chariot every day, and where if you dug down far enough you'd find the underworld. A world where the Gods are real.
That's Glorantha. And your character lives there, in a world of mysteries and secrets, where much of the knowledge of the past has been wiped out, and then reinvented. Is what he knows true? Was the god he worships always a god, 'cos there are some intriguing hints he may once have been something else - maybe even just a mortal. Could your character follow his path?
Glorantha has been in existence close on 40 years, first the work entirely of one man, and now the work of many, in a loose collaboration "chaired" and "guided" by the same man - Greg Stafford. You can contribute to the "reality" of Glorantha - cultures, histories, mythologies - if you want, or you can just draw on the product of the creative energies of hundreds of passionate players & fans worldwide for a game world of really amazing depth.
If you want to pile down a dungeon and smash up some monsters, fine - Glorantha has that. If you want to track down the mysteries of the past, or destroy an Unfathomable Evil, or raid the lands of the gods, Glorantha has that, too. And if you want to experiment with your ideas on cultures, mythologies, and so on, Glorantha draws on the works of such key thinkers as Joseph Campbell and so on for its philosophical structure.
Something for everyone, really. Oh, yes - and it owes practically nothing to Tolkien. There are no orcs. It's a bronze age world, inspired more by ancient Sumer, Greece, Rome, and Byzantium than Merrie England or Middle Earth. It's as immediately accessible as Jason and the Argonauts or Sinbad the Sailor - but you can go much, much deeper - if you want.
Told you it was a tricky question...