Ass Pull: While it makes sense how sunlight and water could act as some sort of defibrillator, no explanation is ever given for why blood from resurrected vampires will cure vampires.
It may be that the transformation back left some form of antibody in their blood that others could feed on.
Crosses the Line Twice: The blood substitute test. After the volunteer has a bad reaction but recovers, he merely says, "Ow." He then promptly explodes into bloody chunks, covering everyone with gore. It's horrifying and hilarious at the same time.
The start of Bromley's death is horrific, what with the mass feeding and muffled screams - then a soldier rips off his head and just holds it in front of the camera as the eyes roll upwards. For some reason.
When the vampires who rushed and ate Charlie turn human, they are in turn rushed and devoured by further vampires in a slo-mo orgy of blood and violence. And then the last three surviving soldiers also turn human. If Christopher hadn't charged in just then and killed them all, who knows how long the chain reaction would have gone on?
Chris's friendship with Edward could have been played up to make his betrayal feel stronger.
Senator Turner and the refugees Audrey was harboring suffering from Dropped a Bridge on Him was frustrating, as they could have played a good role in the climax, providing a Big Damn Heroes moment or something.
Allison being turned into a subsider and killed feels this way after her actress's strong performances, given that the only purpose of this seemed to be to give Frankie something horrifying to witness, and he seemed to shrug it off enough to try and turn Elvis again later, while having her survive to get cured by the blood of an e vampire at the end could have been an emotionally satisfying conclusion to her subplot.