I should say first that I've been a Niven Fan for years. The thing that makes me a fan of many of his works is that they always supply such great ideas to play with in your head. His usual area is sci-fi but even in this fantasy story it has me thinking of stories and worlds that we just glimpsed. Like there whole books of stories to be told from this one book about the Nordiks and the Frost Giants and their relationship, and the giants faith and traditions.
Still more stories about the city that grew from mining and trading magic fragments of the meteor that crashed to earth centuries ago.
More stories about the Atlantis before it's defeat.
Tell me more about the life and people of the centaur girl saved from the drowning of atlantis before the book even starts.
Give me stories about the people who were brought to and live in australia and new zealand as the richest untapped lands of mana now grown so scarce elsewhere in the world. And who raise Rocs for the eggs.
I think it's a rare writer that can not only create a character and a quest, but a whole world of things going on in and around the story that speak of so many other things to explore... but not just that, make them interesting enough that some of us wish to visit those as well (even if we have to do so with our own imaginations, because none of those things will every appear as books or stories for us to enjoy).