Everyone "knows" that Frankenstein is the creature, or at least used to. Nowadays it's fairly common in fiction to hear one character snootily correct another about it being the name of the scientist and not the monster. This leads to the quip, "Knowledge is realizing that Frankenstein isn't the monster — wisdom is realizing that Frankenstein is the monster."
An even more pedantic piece of trivia is that technically, there's no "Doctor Frankenstein" in the novel. Victor made the monster as an undergraduate student and never had the time to actually get his doctorate.
I Am Not Shazam: Many people call Frankenstein's Monster "Frankenstein", while he actually has no name. "Frankenstein" is the name of his maker, Victor Frankenstein. But we can probably blame Mary Shelley for that; it would be a lot clearer to all if she'd called her novel "Doctor Frankenstein". This confusion dates back nearly as far as the novel itself, and became established during periods when the actual book was out of print, but its characters and plot were being emulated by stage plays, knockoffs and parodies throughout the pre-copyright 19th century. Ironically, since one could argue that Frankenstein is the "father" of the creature, you could say that the creature's last name is Frankenstein. Mary Shelley referred to the Monster as Adam in letters to her friends, which does make some sense, seeing as it is the first of its kind.
Alternate Aesop Interpretation: The intended Aesop of the story was not to play God, but it is very easy to instead come to the conclusion of much better Aesop being to take responsibility for your actions.
Victor Frankenstein is either a tragic and naive scientist who, in his enthusiasm, bit more off than he could chew and paid a terribly high price for it and has every right to be emotional about it, OR a selfish asshole who tried to keep his reputation in tact by abandoning the monster and got what was coming to him and he's being whiny about things that are entirely his fault.
And the monster is either a far bigger woobie than Victor could ever hope to be, abandoned by the only person he could possibly consider a parent, or a wangsty monster with a Freudian Excuse who needs to take responsibility for his actions.
The monster is nothing but a figment of Victor's imagination, embodying everything bad in himself.
Victor Frankenstein is actually gay and his endeavor to create the idea man was his attempt to make a companion for himself and remove women from the process of reproduction. Saaaayyyy...
Who is the real monster? Is it Victor for be irresponsible and neglectful to his creation and then refusing to create a companion for him, inadvertently leading to the monster killing his friends and family? Or is it the monster, for actually causing the deaths?
In a meta example, Mary Shelley herself. When she gave birth to a premature child, her husband rejected him and instead left for an affair with Shelley's stepsister. Moreover, the character of Victor Frankenstein shares a lot of bio details with Percy Shelley, including his interest for chemistry and his experiments in college. Considering those reasons, many people have come to believe the entire story was actually an elaborated, well-argumented Take That! from Mary to her husband.
Anvilicious: That science in general and creating life specifically is hubris and fundamentally flawed is heavyhandedly hammered into the reader. It doesn't really work; the story doesn't support such a conclusion. A modern, thinking reader is more inclined to conclude "don't abandon your kid, particularly if he is socially handicapped" than "science is hubris" from this story.
Cry for the Devil: The creature's lament at the end of the novel demonstrates that he still has the capability to feel remorse for his actions and that he was formed by his surroundings, rather than born evil.
Draco in Leather Pants: The Creature/Adam Frankenstein/The Monster, whatever you want to call him. In most adaptions of Mary Shelly's novel, he often does get treated much more sympathetically, most notably in the original film, which makes him into a clueless monster which doesn't know its own strength. In the novel itself, he kills a fair number of Victor's loved ones, and is much smarter, perhaps on par with Victor. To be fair, he does feel rather bad at the end, and decides to commit suicide.
Hollywood Homely: It's never made clear exactly why the Creature is so hideous, other than he's big, he has watery eyes, and yellowish skin. Victor designed the Creature to be an ideal human specimen, and the in the earliest illustrations, he looks like Poldark.
Ho Yay: Victor's describes Henry Clerval in a way someone would describe their love interest. Heck, his description of Henry is more thorough than those of his fiancée, Elizabeth. And he reacts to Henry's murder more strongly than Elizabeth's murder, too! When Elizabeth gets murdered, he is 'merely' heartbroken, much like with William. When he find out Henry is dead, he goes into hysterics and is bedridden with a deadly fever for 2 months! This is in addition to the fact that Victor's main beef with his creation is that he wasn't as hot as he wanted him to be. His entire experiment could be read as an attempt to make the perfect male partner for himself.
Not to mention the way that Walton goes on about how perfect Victor is and how he adores him.
Jerkass Woobie: The monster may have killed many people, but he has quite a sad backstory. Victor himself qualifies as well.
In spite of his tragic backstory, it becomes increasingly difficult to sympathize with the creature once he starts racking up the monster points by taking vengeance on Victor's innocent and completely clueless family members.
Victor letting Justine take the fall for William's murder, even though he knows she is innocent and could prove it if he tried, because he fears people might think he is crazy or guilty. Sure, nobody might believe him, but he's a smart guy, he could think of something. This is the point where Victor starts going downhill.
Plot Hole: A more modern viewer would ask why on earth Victor would destroy the female, or just how the wretch knew who Henry Clerval was or his connection to Frankenstein - since the narration does not explain anything.
Slow-Paced Beginning: The first few chapters detail the back story of the sea captain who met the titular Doctor on his expedition to find the North Pole. If you didn't know that the novel was a Story Within a Story (Within a Story) you would read the opening thinking "Get to the unholy abominations against nature, already!"
There's a long segment where the Monster watches a family for about a year - his yearning for a mate comes from the young man being betrothed to an older woman from some exotic place, as well as his identification with Adam's desire for a mate in Milton's Paradise Lost.
They Wasted a Perfectly Good Plot: Oh come on, a race spawned from the Creature and a manmade Bride would have been awesome. But, alas, Frankenstein had to destroy her body before she could be awakened. Though Shelley was probably more concerned with her story's message than a rad story.
Uncanny Valley: A rare early example - the creature's description is left to our imagination, but since everyone who sees the creature is horrified by its appearance (and proportions), it's not hard to guess why. Mary Shelley wrote about this before people even knew to give the trope a name!
Unintentionally Unsympathetic: The Creature. While it's easy to feel sorry for his sore life since his creation and his long rants about how being shunned leads to being evil are difficult to refute, they expose precisely that he is reflective enough to realize the wrongness of his Freudian Excuse, yet he chooses to become consciously evil.
Values Dissonance: The idea of Victor falling in love with, courting, and marrying his adoptive stepsister can come across as this nowadays.
Wangst: Once Frankenstein starts kicking himself over having made the Creature and the Creature's actions, the emotions he expresses can seem so overwrought that they become Wangst. This is especially true considering his until-now perfect life that only falls apart because of his own stupidity.
Once he made the Creature, almost every inner monologue or conversation is Frankenstein going "woe is me" at length. This goes on for the rest of the book and never stops. It sails past "sympathetic" to just plain "pathetic".
The creature has shades of this as well. Although he is a much more sympathetic character than Victor considering his harsh upbringing, he continues to play the victim even as he's picking off Victor's innocent loved ones. After a while, one can only shed so many tears for his fate.
What an Idiot!: Victor decides to destroy the body of the companion he's making for the creature in order to prevent more monsters from being created. However, he does this right in front of the creature, who we all know is capable of murder.
While the Ito adaptation gives this an explanation, it swaps the idiocy to Henry Clerval instead. Due to the female wretch being killed in self-defence, he rushes off on his own and is promptly killed by the wretch.
The Woobie: Justine. Pure-hearted, but gets executed for a murder she didn't commit.