I think it important to remember that the superhero is at his essence a libertarian or conservative icon, simply because he uses his 'gifts' and his will to set things right as an individual. Even an anti-system rebel-- say 'Batman' in DK2-- is an individual imposing his or her vision upon the world. The values are individual rather than collective. Moreover, it is the individual, the hero, who is uplifted over the group. One person sets things right, usually with violence. This fits right into the extreme individualist models so common in modern American conservatism or libertarian thought.
Much liberal politics is really based upon working with patience and understanding to 'dissolve' problems, or to attack them at their roots, long before they exist. The teacher who gets an inner city kid interested in engineering prevents a villian, but such stories are difficult to tell in comics and not superhero stuff. Talk is the stuff of liberal politics, or as Winston Churchill once put it, "Jaw, jaw is better than war, war".
So now Winston churchill is a great liberal icon? Huh.
Anyway, this looks a lot like the arguments I've seen here and there that The Incredibles is a conservative/libertarian/Ayn Rand-ian movie. But as both Jesse and this poster point out, the left/right distinction pretty much breaks down here when you poke at it. In their fictional worlds, superheroes are usually one step closer to Nietzsche's Uebermensch (often misleadingly translated as 'superman'), or a lot like Kierkegaard's Knight of Faith; and it's easy to read Nietzsche and Kierkegaard as politically conservative. It also doesn't hurt that, in the mind of today's conservative/libertarian, 'liberal' often equals 'Commie' often equals 'anti-individualism'. But a more nuanced reading of Nietzsche and Kierkegaard, and recalling that Fascism was all about individual sacrifice to build a great society, rejects this simplistic identification.
What Nietzsche and Kierkegaard were all about, at least in this respect, was the rejection of mediocrity, conformity, and the notion that the highest moral duty was to the impartial law. I think this is precisely the spirit we see in all superhero comics (at least, all good ones), and the fundamental tension of vigilantism is the tension of Kierkegaard's Knight of Faith. A conservative can read an elitism into this, like Ayn Rand did when she ripped off the worst ideas from Nietzsche, but a liberal can find a far more democratic dream that everyone can overcome the temptation to conform and deny their own power.
At the end of the Powerpuff Girls Movie, a giant Mojo Jojo threatens the helpless people of Townsville, and tries to convince the Girls that they owe nothing, and can use their powers to help him take control. As they smack him down, the Girls explain that their powers are precisely the reason why they side with the people of Townsville. (Haha, just found a transcript!)
Mojo: For them? The ones who hated you? Have forsaken you? (continues climbing to the building top.) Can’t you see? None of them will ever understand you as I can. For we are kindred spirits, whose powers spring from the same source. So girls – do not make me destroy you, for we are smarter! We are stronger! We are invincible! We have the power! We are superior to them! And we shall rule! All we have to do is work together. Girls! Join me!
Girls (All): Nooooo!
Blossom: We’d never join you! And it’s because… (flying, hits Mojo) … we are stronger!
Bubbles: (hitting Mojo) Because we are invincible!
Buttercup: Because we have the power! (striking Mojo)
Girls (All): We have to protect them from you!
Blossom: It’s you who is to be feared!
Bubbles: ‘Cause you are a monster! (She smashes Mojo’s dome, revealing his brains.)
Buttercup: You are evil!
This isn't a liberal or conservative notion: this is just about doing what's right.