Meh, I thought that the mystery and ambiguity of Miraak made him such a great character, we only have hints of his backstory, and adding the legend of the "traitor" it really seems like time forgot Miraak/
Mystery and ambiguity can make for brilliant characters. One example that I can think of is TIM, whom you can see in my avatar (lets not discuss what happened to this character by the end of the series). However, there still needs to be a foundation of lore from which that character rests. From there, you also need to develop a presence for the character whether they be a hero or a villain. This is precisely where Miraak kind of falls apart. You only see this character twice (unless he steals Dragon souls from you) and the extent of his influence on Tamriel effectively dies when you cleanse the Stones. Bethesda simply didn't give me a sense of why this guy was such bad news or why I should care. Bethesda can tell me he is threat all they like, but words only go so far. Show me what he can do. I felt that they did a good job of this with Herma-Mora, but they practically ignored Miraak.
If anything, I thought Bethesda made Miraak so ambiguous that I could not even take him seriously as a threat. Within a couple of quests, you effectively neuter his plan to enslave Solstheim unless he can physically return to Tamriel. You later convince his Daedric master to aid you in your quest to destroy him, rather easily might I add. Finally, you shout one of his dragon allies into serving you instead, also rather easily. Miraak reeked of arrogance and I saw very little reason for him to be so convinced of his victory given how much damage you did to his plans in such a short time.