In every episode they spend a lot of time giving the characters a reason to look back into their old lives before the lights went out. This is static. In last nights episode, it was the lack of oxygen that allowed them all to hallucinate. Once we knew they were hallucinations it was no longer a suspenseful device.
There were lots of small things that got in our way, but the biggest thing both my wife and I had trouble with was the persistent feeling that the best story wasn't the "present" or the flashbacks, but the skipped over time.