So for me, Nate's character arc (loss of idealism, steadfastness, moral certitude) in the miniseries is so appealing that I -- and I think, to a large degree, our fandom as a whole? -- has latched onto it as the base of his fanon character, although it's in fact what separates him most from his real-life counterpart. I tend to think of IRL Nate as way more conservative, a little hidebound to tradition, very ambitious, very politically aware, someone who's equally at home at fundraisers as he as amongst his books or in uniform. And he's definitely not queer!
And, haha, I think that's the other main point of divergence: IRL Nate never, as far as I can tell, had a very close relationship with Brad. The miniseries created that relationship, and we latch upon it and embellish it-- because don't we want an emotional core of the stories we tell? Personally, I love subverting a privileged upper-middle-class white male military character like Nate by putting him in a beautifully complex gay relationship with someone like Brad, who (in his own way) is far more mould-breaking than Nate ever could be by himself.
by tevere