Mary Shelley's Frankenstein, for some, exists as a warning to two very specific ideas. First, one should be wary of Forbidden Knowledge and the lengths one may pursue in order to surpass previous scientific advancement. Second, the novel serves as a warning about the power of nature.
First, and foremost, the novel serves as a warning against taking one's quest for Forbidden Knowledge too far. Given that Victor, after many trials, comes to reanimate life. Prior to his success, this has yet to be accomplished (or even attempted). Although Victor found success in his experiments, the true outcome of his success proved to be far more costly than the time he spent on his experiments. Instead of being something Victor could celebrate, he choose to alienate his creation, forcing his creation to take matters into his own hands. Without the nurturing care of a "parent," Victor's creation is unleashed on the world unrestrained. This, for Shelley, could show her disdain for things which get out of hand when in search of things which should be beyond mankind's grasp.
Power of Nature
One can certainly see the impact of Coleridge's "The Rime of the Ancient Mariner" on the novel. As detailed in the poem, the power of nature must always be recognized. As for the text, Shelley proves the power of nature in many ways. First, she proves the healing power of nature exists. For example, each time Victor succumbs to illness, it is nature which heals him. Second, the nature of the Creature proves to be far more powerful than his declared war against mankind. It is, essentially, in his nature to be compassionate and loving (which is unfortunately hidden (at times) by his behavior). Last but not least, nature proves to be the most powerful given the death of both Victor and the Creature. While nature has taken its tole on Victor's human body (during his pursuit of the Creature), the Creature gives himself over to nature. In the end, fire (a prominent part of nature and the story) rises over the Creature and his life.
I shall seek the most northern extremity of the globe; I shall collect my funeral pile and consume to ashes this miserable frame, that its remains may afford no light to any curious and unhallowed wretch who would create such another as I have been.
By the end of the novel, Shelley has made her warning obvious: never underestimate the power of nature and never overstep the bounds of mankind.