The combination of dark murder mystery drama and intrigue, and typical Rooster Teeth-style comedy. Some think it works out well, others think it renders the whole thing unrealistic (or just bad) due to everyone's reactions and conversations. Of those in the second camp, some wish it was taken completely seriously, others wish it was all comedic, and some just wish the balance between the two was done better.
The actual Audience Participation is either well done or not fair; the clues tend to be vague in regards to what actually happens (for example, for "one ran out of air", the death was Adam being strangled by a computer mouse cord, which is quite a leap in logic; plus there was the whole "martyr" issue below under Critical Research Failure). This annoyed many people, especially when Word of God made a post explaining the reasoning behind each line of poetry and the resulting death, with many people shocked they were expected to make those connections. Of course, it's up in the air whether these people are correct, or just not very attentive.
The fact that one of the guesses was to, after only the very first episode, determine both the killer and the eventual survivor, also annoyed people as time went on, due to it turning out there was only one clue for each that were both very vague: Barbara's photographed victim, Blaine, died of poison, the way Michael did in the first episode; and Chris quotes a line from a work of Tolkien for which the full quote (not given until the finale) ends in "the crownless again shall be king" to foreshadow Mad King Ryan's eventual survival. And there were loads of Red Herrings throughout that episode as well. Many fans saw this as just Rooster Teeth rewarding people who made lucky random guesses.
Critical Research Failure: The poem states "One was martyred" for the third death. The definition of being martyred is, to put it simply: being killed for your beliefs, usually in a religious sense. That... isn't how it goes when the third death comes around. How the killing goes is as follows: Chris is killed Taking the Bullet for Burnie, because apparently the writers think being martyred means pulling a Heroic Sacrifice. The only way you can consider this accurate is if you assume Chris was killed as some result of his Lord of the Rings costume and infatuation, but even if that was the case, it doesn't come across well at all.
Fridge Brilliance: Why is the killer using Ryan's GTAV costume? It's one of the three outfits from the Achievement Hunter series of Let's Plays that has a mask that covers the whole face, and who would want to lug a heavy Spartan suit around or don biker apparel to murder people? Anonymity and convenience! As explained in the entry below, Barbara intended on framing Ryan to be the killer, and what better way to do so than dressing in a costume of a character well-known for being a video game sociopath?
Nearly everything Barbara does becomes this after it's revealed that she is the killer.
Fridge Logic: The poem implies the killer is planning his or her own death. Explained in the finale. Barbara had intended to frame Ryan as 'the killer' and make it out as the supposed 'innocent survivor.'
"Funny Aneurysm" Moment: In episode 6, Miles finds what appears to be a mocap video of what he calls "dot sex" (It's actually Adam being strangled and his futile fight to save himself). As he does, Miles walks away in shame and proclaims "You're a sick, sick man, Monty Oum." A month later, Monty would be sick... and pass away.
Idiot Plot: Nearly all of the deaths could have been avoided if the victims had just used a little bit of logic:
If they had been paying attention to their surrounding, the Gavins wouldn't have collided and fallen onto the mousetraps.
Burnie would have known about the killer and might have even killed them if Chris hadn't been such a Cloud Cuckoo Lander and tried to kill the murderer themselves.
To be fair though, Burnie could have moved to another part of the room where Chris wasn't between him and the murderer and could have shot the murderer from there instead of wasting time letting Chris fool around thus allowing the murderer to escape.
If Burnie had kept Chris's magical sword of detect evil after Chris died and the killer escaped then he would have been able to work out who the killer was on his own. Instead he, for some reason, leaves this extremely useful tool behind with Chris's body.
If Burnie had been paying attention they would have noticed the scorpion on the table.
If Adam hadn't left that pitchfork behind (itself an extremely stupid idea, since they had no other weapon and a killer was on the loose) then not only might they have had a chance at surviving, it would never have been used to kill...
Gus, who could have just closed the door on the killer.
If Adam had actually teamed up with somebody, the killer wouldn't have ambushed them as easily. Also, whispering isn't exactly the best way to grab somebody's attention.
After luring the puma away from Ryan, Lindsay could've just made a run for it, instead of trying to make friends with the puma.
Miles had the right idea in putting his feet off the treadmill. However, he doesn't do that after the first attempt.
Barbara might have been able to kill Ryan before he recognized her explosive-filled Creeper suit, had she kept using the sword instead of discarding it for no apparent reason and continuing the fight with sausages.
And even Ryan took far too long to remember that he had opposable thumbs. Though possibly justified as he may have been worried about the broken glass cutting him and was hoping there'd be a way to open the hole up without having to smash through it, so waited until it was a last resort.
Signature Scene: For many, Burnie's death. The Lock-and-Load Montage, the following Mood Whiplash when he's killed by a scorpion sting, and the Red vs. Blue music as the camera pans up from the scorpion to a picture of the scorpion tank from Halo, revealing exactly what just happened, all come together to make a memorable moment.
The sequence of Burnie narrating the poem in the first episode.
Spoiled by the Format: Ryan starts losing oxygen whilst trapped in a hole in episode 6 and their framed picture appears on the wall to show that they have died...with a good two minutes left to go in the episode. Not surprisingly, the picture falls off the wall and Ryan is revealed to be alive, with someone else's picture replacing theirs on the wall at the end of the episode, this time hammered to the wall to make sure it stays up.
Ultimately the identity of the killer. By the end of the second-to-last episode, two of the remaining survivors were already eliminated from suspicion due to either being otherwise busy while there was a killing (Ryan) or was actually on screen alongside the killer (Miles), meaning Barbara, by logic, had to be the killer. The last episode seems to lampshade this, as she introduces herself as the killer matter-of-factly.
Ryan is falling into this as well. He believes he's somehow responsible for Michael and Gavin's deaths and is shown having a mental breakdown as a result. He's desperately trying to convince himself he's not the killer and even hallucinates someone in a Creeper suit attacking him. Many fans soon began hoping that he'll be the one who'll survive this ordeal after the episode with this scene. They got their wish.