Balrogs *don't* have wings--such has always been my take, so I shall put that here just in case the old coal still has enough fire to kindle some reaction.
The Book of Mazarbul, though I think its presentation would appeal to a wide variety of readers, really is the sort of passage that tells you about its author: this is a damaged, historically important manuscript, with loving detail lavished upon it by Tolkien--to the point of making facsimile. Most other authors would not have gone to such detail--and, if they had a Book of Mazarbul at all, I doubt this would be its last appearance: sending it off with Gimli would be a Chekhov's Gun for a later revelation of... something.
A detail I don't remember from my previous readings is Boromir's horn-call, an event with a definite force here. It gives us a trifecta of horn-calls from Boromir: a heroically brash one on the departure from Rivendell, a heroically defiant one here, and a heroically tragic one at Amon Hen.
It also struck me that, wolves or Nazgûl aside, this is the first
battle we see Aragorn in, and he's clearly a badass with a mighty sword. Frodo is too, though Tolkien's slyer about that: Frodo's strike against the troll is more effective than Boromir's, and his decision to strike in the first place is fearless: not what you necessarily expect from hobbits, and something to remember much later when considering the later Frodo's pacifism.