Way underdeveloped. There should have been more books about him, or the one book should have been longer. He should have had more dialogue. Something, anything!
Agreed. While I think the buildup was magnificent, with him casually knocking you out of Apocrypha and swooping in to steal your Dragon souls, the payoff was disappointing. I also point to the relative shortness of the DLC's MQ as a (not the) cause. As I've been prone to say, despite the panicked cries of "video games are too long so people won't play them anymore!" if someone buys a TES game they both want and expect a timesink.
Let's look back in time at Morrowind. A great deal of early quests and a few midgame ones existed almost entirely to set the stage, to give you well-woven exposition into the nature of Morrowind, the Tribunal and the Sixth House so I, the player, actually cared about the conflict between these groups and what it meant for the land and its people. Seriously, there was a major early quest spent entirely gathering information for Caius Cosades that was secretly a way to exposit to the player without being clunky. It made sense for the story and served a narratiive function. If there is one major thing that could be improved in Skyrim and all it's questlines and DLC, it is this. They jump into the conclusions of the quests without giving us enough reason, unless you've played the other games and already fallen in love with Tamriel, to care about what happens. While there was an information-gathering quest in Skyrim's MQ, it was just one and very narrow in its focus, giving information on the Blades and only scraps of anything else. Imagine if there were a second quest to gather information specifically on the dragons, and a third to gather information on the Blades that was needed to find Sky Haven, rather than one lump quest that was to find Esbern, who knew all this stuff but wasn't really sharing it.
Similarly, look at Dragonborn. How much bettwe would it have been if the Telvanni quests wove into the MQ, and Neloth was both the Guide and the Threshold Guardian. You would need to find information so he could help you, and in so doing a player who wished to do so could take and anolyze this information (on Miraak, Apocrypha, ancient Solstheim history or anything else, really) him or herself.
Yeah, that came off harsh. I like Miraak, and the DLC as a whole, but there is definitely room for improvement in the "develop stuff pls" department.
EDIT: Also, his status as the First Dragonborn had zero relevance to the plot. That's pretty low, dudes.