KNF is actually the first book I read from the Horus Heresy series, and I have to say that it's still my favorite. It's a book about lonely "What you are in the dark" heroism in the face of annihilation and touches the heart strings at the right moments - to the point that one of the biggest heroes in the novel does not even get to say a single word.
The contrast between Guilliman and Horus (from the first few books) is actually pretty stark, with the former clearly leading as part of a (cold) military formula, while the latter leading through the sheer force of his own personality. That's why the Luna Wolves essentially fall apart and panic once Horus gets wounded (and they do the monumentally stupid act of turning him over to Erebus), whereas the Ultramarines react to the possibility of losing Guilliman with "There was no chain of command, so I decided I had to build one myself."
And I have to say, I now get the Ultramarines after reading KNF. They're stick-in-the-asses, but that's because everyone around them are either unreliable (i.e. White Scars, Luna Wolves) or verging on the downright psychotic (Space Wolves, World Eaters). And they have to be the adults because no one else will; certainly not the Emperor. And that fact that Guilliman knows he's the mature one (and makes no attempt to hide it) rankles on his brothers.
By contrast, Horus (from Horus Rising) thrives off making the people around him feel good - partly as part of his leadership style, but also because he has this need to be loved and adored. The latter is what causes his many moments of self-doubt, to the point that he even thinks that Sanguinius should have been Warmaster.
Finally, a peeve: I really don't get why the fandom really likes the Loyalist Luna Wolves that much. Sure, Loken and Tarik are great in Horus Rising and he's very human for a Space Marine. But he basically does nothing along with Tarik for the next two novels, despite KNOWING that a purge is going on. They seem to have turned off their common sense in favor of constantly telling themselves that "Astartes shooting Astartes will never happen! We're safe!" (despite plenty of evidence otherwise, as Russ would happily tell you)