These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Monsters vs. Aliens
Tropes for the movie
Badass Decay: In-universe: the Missing Link apparently once held his own against the navy, the national guard and some baywatchers. Until he beats up about thirty clones in hand-to-tentacle combat, that's very hard to believe.
According to his website profile, he was a real badass. Until 30 years ago. Probably out of his prime at that point.
Not to mention he's been cooped up in Area 52 for several decades. Note the beer belly.
In the TV series they seem to have had him working out in the interim. Not much more effective, but less beer belly.
Cargo Ship: B.O.B./Green Jello. A match made in mixers.
Kaite and Cuthbert, the One-Scene Wonder role reversal teens who encounter the aliens early in the movie.
Inferred Holocaust: In a post-credits scene, the President accidentally launches all of the nukes.
Ship-to-Ship Combat: The fandom is a bit split when it comes to who Susan gets paired with. Given the lack of Token Romance within the film, many tend to gravitate towards Dr. Cockroach/Susan since he takes an active role in trying to reduce her size and they seem to converse the most (also, they're the only two monsters who used to be human). But Link/Susan has its share of fans as well, considering that Link's whole deal is that he chases beautiful babes anyway. God help you if you're on the fence...
The whole ship-to-ship combat situation for this fandom is not that bad. There are at least fans who can tolerate both points of view on the matter no matter which side they're on (if they're even on one in the first place). There are plenty of fandoms that are much worse..
Shout-Out: The whole plot premise could be considered a shout out to the Wizard of Oz movie. Think about it. A MacGuffin girl (Susan/Dorothy) taken out of her normal life and set on a quest to defeat a Big Bad (Gallaxhar/the Wicked Witch) via clingy Phlebotinum (Ruby Slippers/Quantonium) with three male friends who double as a power trio (Link, Dr. Cockroach, B.O.B./Cowardly Lion, Scarecrow, Tin Man) plus a pet-like animal (Toto/Insectosaurus), is captured by the Big Bad so needs to be rescued by said friends who do so by pretending to be part of the Legions of Terror (complete in Monsters with B.O.B.'s almost direct "I may not have a brain but I have a plan" quote).
Ugly Cute: Pretty much all of the other monsters aside from Susan herself.
Uncanny Valley: The stylized inhuman "Monsters" are a bit easier to watch than the actual humans.
To paraphrase the movie description at the top of the page, aside from the monsters, all the humans basically look like live action versions of the humans you'd see in mags like Cracked or Mad. Kinda off-putting at first, but you get used to it.
Viewer Gender Confusion: Insectosaurus. The movie calls the monster a "he", but a behind-the-scenes book lists the monster as a female. This has lead to a Broken Base amongst the fans of the movie as to which is truly canon regarding the monster's gender.
If Insectosaurus was only a larva before she metamorphosized, then it may not have even had a gender or anatomical indicators thereof. Calling it "he", as with B.O.B., could've been purely a convenience.
What an Idiot: Derek dumps Susan for the sake of his career without considering the worldwide fame their continued relationship would bring him, not to mention the millions of dollars in book deals and movie rights. Sure, he'd need a raincoat to have any hope of ever pleasing her, but it's still a very stupid decision from someone who's already been characterized as a selfish asshole. He does put things together, sort of, when he forgives her and offers to take her back at the end. Of course mentioning that he was offered a network slot if he got an exclusive interview in the next sentence made Susan doubt his sincerity. Letting that detail slip only reinforced his stupidity.
Susan: Are the cameras running, sweetie? Derek: Why, of course they are! Viewers (facepalm)
According to the commentary, one possible (deleted) follow-up to Derek dumping Susan has her going on a rampage through town to try and crush Derek. It would have been somewhat out of character for Susan to do this at this point of the movie, but that doesn't remove the fact that she could have done it, thus further reinforcing Derek's idiocy.
The Woobie: Susan. After Derek dumped her, she looked so much like a kicked puppy.
"I don't want a poster. I want a real kitten, hanging from a real tree."
The monsters, after they scare off the party.
Link. The poor guy gets his butt kicked several times during the battle against the robot probe, and even gets knocked out after it's already been defeated. He was so obviously downbeat about his performance that one almost expected him to do a Face-Heel Turn.
Tropes for the series
Designated Hero: Most of the time the titular monsters come across as idiotic xenophobic jerks, and there are several occasions where the conflict of an episode brought about by the monsters being assholes or just being plain stupid.
Hilarious in Hindsight: The Internet (who is actually an alien) constantly uses old and dead memes. This would be a groan-inducing attempt at being relevant...if not for the brief resurgence of extremely old memes at the time, making the episode look relevant and seem like a parody of current events.