Allegory: Prometheus
As discussed in Major Themes, Victor Frankenstein is allegory for the mythical figure, Prometheus. As the latter stole fire from the gods and was punished for it, so did the former discover the secret to creating life, and subsequently suffer for it.
Motif: poetry
Shelley intersperses quotations from and references to poetry throughout the novel, adding a level of artistic awareness to a novel that purports to be testimonial in nature.
Allegory: Genesis
Frankenstein's creation of the monster can be read as an allegory for the creation story from Genesis, of God creating Adam. As is the case in that story, Frankenstein forms the creature in his image (i.e., that of a human -- albeit grotesquely), and animates the creation.
Motif: letters
The novel is deeply concerned with evidence and direct testimony with respect to events. As such, proof of communication between people is often conveyed in the form of letters, both within the story (e.g., the monster showing letters from his upbringing to Frankenstein) and in a reflexive context (i.e., the most direct interface between the reader and narrative is the collection of Walton's letters to his sister).
Motif: retrospection
The novel is explicitly retrospective on every level: Walton is recounting events that have already happened in his letters to his sister; Frankenstein is recounting his history to Walton; the monster is recounting his past to Frankenstein. As such, the tone of the narrative is generally very self-aware and reflective -- expressing regret over what happened; imagining how events might have gone differently; and so forth.
