Being one of the people left behind as California, Las Vegas, Washington, D.C., etc. all succumb. The LA footage, particularly, reminds of 9/11 (collapsing office buildings with people inside).
The LA subway train falling into one of the chasms: imagine being on that train.
Being on the streets of LA while the earthquake is going on. A giant crack appears along the ground and the dirt beneath your feet starts tilting, causing cars to come hurtling towards you at apparently terminal velocity. In the trailers that show that clip, one of the people knocked into the chasm by a falling car SCREAMS.
Basically being anywhere on Earth at any point in time. If the earthquakes and volcanoes don't get you, you can bet the tsunamis will.
Not being on Earth could possibly be even worse because there would likely be no way to contact Earth.
If you're still left alive after the caldera goes off (which will pretty much choke the living shit out of anyone in America, immediately kill any person in the Yellowstone area, and so on), you get front-row seats of a tidal wave going to drown you!
Falling to your death. Too many times to count.
In the scene where the plane in Las Vegas takes off, the guy on the ATC lets out a chilling, painful scream as he is fried alive inside the control tower when the ash cloud hits it.
Imagine making a narrow escape from the disasters that hit your area, arriving at the rendezvous point for your rescue aircraft just in the nick of time with your family, and being flown to the Himalayas... only to make the horrifying discovery that your assigned Ark was one of the ones that was damaged. I mean, yeah they eventually did manage to squeeze everyone onto the remaining Arks, but still can you imagine the absolute terror of being in that situation? As if that is not bad enough, imagine finally being told to go to the boarding bridge, ending up on the edge, and then going over as the crowd just keeps on shoving while the bridge lowers into view.
Charlie's whole speech about what would happen prior to the destruction of Earth (global economic turmoil, war, genocide, etc.) is particularly unnerving.
Tony's phone call to his son. He hears the dying screams of his granddaughter before the line goes dead.
Imagine being a person on board the cruise ship as it capsizes. If you somehow survive, you now face the nightmare of being trapped in an upside down boat that will inevitably sink. And because the mainland has been utterly devastated, there's no chance at a rescue whatsoever. No one is coming to save you. Almost makes death by volcano/earthquake/tidal wave seem like a relief, because at least it'd be over quickly.
Hawaii in flames. There is absolutely nowhere to land because everything is burning.
The roaring noise when the quakes hit Los Angeles and Washington DC sound like an angry monster is attacking.