There's a decidedly delicate balance between scoring a film and being a musician. Structure is dictated by scene instead of theme so in the most part the scorer's niche is already defined by someone else. Secondly and crucially, the conceptual component to your output has a target goal before instrument is picked up (or project file opened).
Composers writing for films don't get to start with the traditional clean slate, and often are just there to add in brushstrokes to the film around the plot and visual devices; this is why so many films have forgettable soundtracks. Reznor and Ross don't really take this tact. The stretching of certain barriers has become a device in the defining of key filmic moments, notably in the use of volume; moments in the film see the soundtrack being the focal point, straying from the traditional 2.5 mix-concept (dialogue, foley, score). Thanks to Michael Patterson for superb mixing. But in approaching the composure of the soundtrack similar to that of their traditional work, albeit with an altered dynamic, the validity of the pieces seems much more natural. Unlike many modern electronic/orchestral hybrid scores, the tracks stand up on their own without need of visual accompaniment; to put it bluntly, it's much more interesting than their peers' output seem to be. If this were released as a traditional album it would certainly receive positive feedback, and the fact that it's been composed with enough dynamics and space to allow it to operate along side a film shows the maturity and a deeply developed set of skills that Reznor and Ross posses at their disposal.
Instrument usage by the pair is fairly predictable knowing their prior recorded output; dense synth, strings, piano and the occasional guitar ebb and flow in and out giving the pieces a feeling of life, like the anxiety-filled breathing of socially awkward Mark Zuckerberg. Careful selection of instruments definitely has helped add to the overall pace of the soundtrack, a notable example is the guitar in Intriguing Possibilities
, and again the bassy, Deadmau5-like synth stabs in Carbon Prevails
. The addition of a remixed and choppy In The Hall of The Mountain King
bringing home the grandiosity of the story.
Reznor and Ross have perfectly captured the suffocating atmosphere of having five-hundred-million friends, and none of them being close to you at all. Further to this, they've delicately crafted a soundtrack to a film that breaches the gap between background and foreground; not just supporting the film sonically, but creating a collection of music that stands on its own.
This is an example of how things should be done.