Great question! I would say that all novels, not just fantasy, find their strength in their characters. Plot and setting are also important (especially for a fantasy) but if you don't have likable characters, no one is going to care how much time you put into the geography. Give your characters depth; figure out why they're doing what they're doing and it will make their interactions that much more memorable and intriguing.
Other than characters, I would say that believability is important in a fantasy novel. It needs to make sense why everything is the way it is. Are there castle ruins in this scene because it made a cool backdrop for your characters, or is it there as a horrifying reminder of the war a few years earlier? Ask yourself; would this still be here if the characters never saw it? Once your world exists beyond the bounds of your story, then it will sound more alive and believable to your readers, even if they aren't seeing the full scope of it in your novel.