Skimming the decision, I'm struck by how SCOTUS broke with the custom of deciding cases as narrowly as possible. The genius of the Supreme Court is that they usually avoid actually making big decisions. This is a good thing; nobody really wants them to legislate from the bench. They decide cases as narrowly as possible. There was a harbinger of this decision when it was announced that SCOTUS would expand the scope of this case to campaign finance law as a whole, and not just the question of whether or not the movie Hilary could be shown in the days before an election.
Stevens points out in his dissent that:
Moreover, even in its merits briefing, when Citizens United injected its request to overrule Austin, it never sought a declaration that §203 was facially unconstitutional as to all corporations and unions; instead it argued only that the statute could not be applied to it because it was "funded overwhelmingly by individuals.”
This is not merely a technical defect in the Court’s decision. The unnecessary resort to a facial inquiry “run[s] contrary to the fundamental principle of judicial restraint that courts should neither anticipate a question of constitutional law in advance of the necessity of deciding it nor formulate a rule of constitutional law broader than is required by the precise facts to which it is to be applied.” Washington State Grange, 552 U. S., at 450 (internal quotation marks omitted).
It is worth pointing out that corporations were able to take part in campaigning in the past, they just had to do it through PACs. Apparently, even that small hoop has been ruled unconstitutional, under the First Amendment.
Sen. Feingold has already registered his displeasure with this possible outcome. Let's see if Sen. McCain still has a principled bone left in his body.
EDIT: SCOTUSblog has a good look at what questions the decision answers, and what questions are left unanswered.