I'm not going by the common law. I'm just trying to apply it reasonably... I would define it as expression, but I think it applies less on public property than it does on private property.
I see. That seems like a very reasonable view. For example, I hardly think that walking nude on government property is protected "speech."
I would say that the First Amendment prohibits the government from punishing or restricting any expression whatsoever on the basis of content. The government can certainly impose reasonable restrictions on the time, place, and manner of delivery; for example, it may prohibit a person from causing a public disturbance by blaring his message on loudspakers in the middle of the night. Or, to use your example, the government can prohibit the putting up of a political sign on government property. However, any time, place, and manner restrictions must be completely independent of content.
I think that the Supreme Court has not gone far enough in protecting the freedom of speech in some cases. In particular, I am rather appalled that anything that is "obscene" or does not conform to "community standards" can be banned. On the other hand, the court has expanded it too much in other instances. Most notable, I think, is
New York Times v. Sullivan and the creation of an "actual malice" standard in libel cases. (The First Amendment absolutist Justice Black makes an interesting case that libel laws are unconstitutional, but I am not wholly convinced.)