Discuss different potential origins of super powers, such as inborn, mutation, scientific, accidental, magical, etc. How does the way that powers are attained affect the tone of the story? What about stories (such has SG) that allow for different power origins in the same story?
The origin of super abilities is important in developing the depth of the main character(s). The importance of what that origin may be is dependent upon the hero in question. Iron Man, for example, was created by a tech genius. However, it makes for a much better story that Iron Man was created after Tony Stark survived a life-changing hardship, than if he decided to make a suit of armor and be a hero because he was just bored of getting girls by being rich.
Thor spin-off character Thunderstrike was granted his powers after what could be considered a trial by ordeal. The back-story is important for him because it explains not only his powers origin, but also his affiliations and alignment as a hero. In contrast, Spider Man is granted his power set by accident. He has no history of being a hero, or even really wanting to be one, so the exact nature of how his powers were granted is important for his hero theme, what is of more importance is how he matures and comes to understand the ethical considerations of having powers.
In many ways, being born with powers is a shortcut for a story writer, because by the time the character is old enough to be a hero or villain he, or she, has already decided their world views, alignments, and goals. This would let a writer leap into the action of the story, revisiting the back-story in flashbacks only when they must to make the story complete.
How the hero gets her/his powers is one of the key factors in the tone of the story. First it sets the 'rules' of that world universe. Take two heroes who have the same sort of power origin 'Shazam' and 'Doctor Strange.' The former is a instant gift to the pure of heart. The later is the result of self-discovery and redemption. In Shazam's world, magic is karma's way of helping Billy out. In Doctor Strange, magic is a mark of dedication and practice.
Another important element of 'how-I-got-my-powers' is establishing the hero's motivations for being a hero. The X-Men are fighting for their race's survival. Spider-Man is doing a life-long penance to ease his guilt. Batman's sadness and rage have turned him into a shield for Gotham. All of these set the tone of the plot.
A story like Sidekick Girl is written to be flexible, so different arcs can have different tones and power origins. For example the Coroner arc is quiet dark and gritty, but the Body Switched arc is pure comedy. The villains and characters introduced give the reader a 'heads up' to what's coming. I personal like this style because life's tone varies just as quickly.
However, I disagree with rypperd0c about inborn powers being a shortcut. I find it harder to write because more character building and back story is needed to explain worldview. Instant powers often occurs in a crisis were the reader can learn about the hero's character (example: Bruce Banner sacrificing himself and getting radiated.) The 'I'll be a hero' moment for Val is a little murky, but it's clear something helped her make that decision.
There are many ways to gain superpowers. Be it trauma, mutation, radiation, exotic formulae, hard work, magic, freak accidents in a lab, use of technology etc. Now to some, the inclusion of technology and hard work may not feel justified, but if Batman has superpowers, then this is so. If Iron man has superpowers, then this is so. If Mecha Maid, Charles Atlas, Dauntless, all the Tinkers of Toybox, have superpowers, then this is so. So, then, this is. The origins of powers dictate the manner in which powers are perceived. Mutations allow for a very telling tale of discrimination, which can mirror another phenomena that has gripped the religious right for fifty years (as shown in X-Men). If superpowers are induced through trauma (a la Worm) then the setting with have a vastly larger number of villains than heroes, as well as a simple fact that the majority of heroes will not be entirely mentally stable, not in the least because their "trigger event" is defined as the worst moment in the person's life. For stories with multiple mixed origins, the use of different origins means the setting isn't as influenced by the way superpowers are gained, simply because there is no single unified cause that is common in every superpowered. This is not a bad thing, but rather an equalizer, allowing for a wider range of cast to partake in the game of Cops and Robbers we know as superpowers.
SuperHero Alias: The Baron.
Powers: Teleport objects and entities through shadows.