The intro the movie involves a bunch of shots of paintings from either the American Revolution or the American Civil War. At first glance, this just seems like a pretty cool background to set the Kickassia theme song to. Except the majority of the film's plot revolves around a civil war - so the various shots of paintings of two big civil wars instead become a pretty slick foreshadowing of what's going to happen later on in the movie.
Similarly, the end credits are a stylized reproduction of the Bayeux Tapestry, which illustrated the invasion of Britain in 1066. Since the movie was about the invasion of an independent nation, this was fitting, although it's uncertain how many of the viewers actually understood the reference.
The shirt Spoony wears in Part 4 says "When I play doctor, I play to win." Later on in that same part, he becomes Doctor Insano.
The reason no one's acting like they do in the videos because this is real. Take Film Brain, for example, writes himself as sarcastic on his show but we're not seeing him on his show. He might be a bubbly guy who writes himself as a Deadpan Snarker.
However, they are calling each other by their reviewing names as though those were their real names....
Perhaps because not enough viewers would recognize their real names?
Later videos have established that yes, their actual first names are their names in canon as well.
This might be a different layer of reality. This is how the characters, in-universe, act when they're not in front of a camera. It's not just Doug Walker playing the Nostalgia Critic, it's Doug Walker playing Doug Walker playing the Nostalgia Critic.
Perhaps Santa Christ, a Christian icon, wasn't resurrected immediately because That Jewish Guy refused to say he believed in him.
Also, why wasn't The Distressed Watcher among the cameos that tried to resurrect Santa Christ? Well...
Linkara seems like a total dick when you take Dr. Insano's defeat at face value—he advocates the Critic finishing him off, whereas Benzaie wants the Critic to let him live because "technically it's still Spoony in there." However, when you take into account that Linkara has cloned Spoony so effectively there's no difference between the end result and outright resurrection, it makes sense. Add the fact that Linkara could also reprogram a new Clone!Spoony's mind to get rid of the Insano alter-ego and killing Spoony!Insano seems like a good idea.
Something that's helped on by later Critic episodes; the reason why he wanted power so badly and was quite happy never to do reviews again? Reviewing quite often makes his life even more miserable.
In Episode 2, Sage takes cover from the cannon fire behind a rock that's much too small to protect him. How did he survive? He was holding an uzi.
Seeing as how they've mentioned it a couple of times in other crossovers (and its been the only one to be mentioned), Critic willing to kill Spoony-inside-Insano makes a lot more sense if you get that he's more than likely still hurting over "Spooning With Spoony 2".
Notice how Spoony was so distraught about becoming Insano again, was carrying a Laser Sword (Or if you prefer, a weapon of science) instead of his Final Fantasy VIII Gunblade, and Spoony has never made reference to the events of Kickassia yet Insano has? That's because it wasn't Spoony in Kickassia, but Insano had managed to suppress "the darkness".
Wild Mass Guessing. The idea is also put forth here.
Even after conquering Molassia, all the Nostalgia Critic does is watch old cheesy TV shows, implying that his original job is pretty much the only thing he's good at.
During the invasion of Molassia, Jew Wario inexplicably adopts the personality of Yanki J. (long enough to use his trademark baseball bat to hit back cannonball fire), then changes back to himself when he falls on the railroad tracks. During a later storyline on "You Can Play This", we find out that JewWario belongs to a line of mystical warriors known as the Warion, and as such has the ability to channel the personalities and abilities of his predecessors—including Yanki J.
This especially makes sense given that at the time Kickassia took place, Jew Wario was unaware of his powers; he could have unknowingly tapped into them during a moment of anxiety.
Little Miss Gamer left TGWTG shortly after Kickassia. When the Critic calls to tell her IT'S TIME, she's been sucked into her NES and is fighting off videogame enemies. That brief segment marked the final appearance of her character; there was no farewell video, and no attempt to "write her off" the site. Ostensibly, this means that Little Miss Gamer is still trapped inside the game—probably forever.
She was shown as just fine during the "I Believe in Santa Christ" Montage, so presumably she just got used to being the protector of the NES realm, like Captain N.
During the Santa Christ scene the Mother Brain can actually be seen exploding in the background, so it's possible that she fulfilled her mission and was able to leave.
Or maybe she stayed and became their Queen.
When Sage pops up in the Critic's show later on, he first appears as the maybe-devil who has Critic promise to give him everything, and gets even creepier in the next appearance, perving that if Critic dies he'd have nobody to play with. That in mind, what's the betting that Critic can eventually look forward to all those lovably demented methods of torture that Sage came up here?
And speaking of later Critic episodes, it's been established that Critic can't wear his usual suit when he's really hating himself. He spends most of this movie psychotic and in an M. Bison costume...