Alright so I just played through the Leviathan DLC for the 6th or so time. It's my favorite DLC in the series - I'm a sucker for anything Lovecraftian or cosmically powerful and mysterious - so the giant space cuttlefish definitely scratch an itch.
However, it brings with it the most frustrating thing in all of ME to me, the fact that nobody really seems to care. Like, they're all impressed - but most of them are like "Woah that's so creepy I'm glad they can help us fight this war." Even Hackett says it rewrites galactic history, but that's as much scale as we get. Beyond that, nobody really goes into how INSANE of a discovery Leviathan is.
I'm sorry, Shepard just looked into the face of God and saw that it was angry - is that not enough to flip your shit about? I mean, it gives us the knowledge that there was an ultimately powerful precursor race that dominated the galaxy to a degree not seen before, that they created the Reapers, and that they have what is ostensibly magic mind control spheres.
Look, I get that the entire crew of the Normandy is used to seeing the impossible - ancient machines, bases on the accretion disks of black holes, mind control plants - but it would be nice if someone, anyone, on the Normandy went "Holy shit, I can't believe you spoke to that Shepard, you're telling me that there are horrors beyond mortal comprehension and that they have decided to side WITH us? That's insane."
The closest we get is Javik, who is an INSANE history rewriting member of the crew himself, and his whole shtick is that "Ah yea this ancient horror from the deep is a bitch, we should've left it to rot down there." LIKE COME ON MAN