Note: The following contains spoilers.
REC 2 starts out very much as the Aliens to REC’s Alien. Meaning that the original was a group of ordinary people shoved into extraordinary circumstances that they have no real hope of overcoming and they get picked off one-by-one, while the sequel starts with an armed, highly-trained group (in this case, a GEO team) going in after the original carnage to clean up. Some of the tension of the original movie is lost because of the body armor and the machine guns, but throughout the first half of the movie things remain a solid zombie movie with an interesting, religious frame for the outbreak.
Then the movie decides to switch POVs halfway through and things slam to a screeching halt. I might understand why going back to a group of relatively helpless people would be good for upping the tension, but this was just a miscalculation on all levels. The husband/dad of the people inside the building barely does anything, the firefighter is the only likable one despite not having much of a personality because he’s actually doing things, and all three of the kids are just flimsy, boring characters who are only there because their motivation is “Hey, let’s do this!” You could completely remove them from the story and have a neater, tighter movie. But it would have only been 50 minutes long, so of course a B-plot had to be inserted.