“Obviously.” In my view, the word “obviously” should only be used in arguments when the point asserted is generally beyond debate. Consider an example. When discussing judicial nominations, you could say, “Obviously, Obama has the constitutional authority to nominate anyone he wants.” The word signifies that the point is a shared view that isn’t genuinely controversial. It helps the reader by announcing that you’re just establishing common ground.
“Obviously” tend to be an unpersuasive word because it is rarely used this way. Especially on the Internet. Most of the time, the word is used when the author has strong feelings. A view passionately held becomes so central to the speaker that it seems obvious to him. For example, “Obama is bad for America” becomes “Obama is obviously bad for America.” This usage usually backfires because it suggests the author doesn’t know or care about the views of anyone else. That is, the speaker is so wrapped up in his own perspective that he confuses the intensity of his own belief with its persuasiveness to others. Folks who are closed to the views of others don’t make very good advocates: The come off as cranks. So use “obviously” with caution.
UPDATE: Several commenters bring up the use of “obviously” in legal arguments specifically, especially in briefs. It’s much the same problem, with a slight twist: Some lawyers use words like “obviously” and “clearly” in briefs out of a sense that perhaps their apparent personal conviction and spirit will itself persuade the court. If a lawyer feels so strongly, the thinking goes, perhaps the court will assume that there must be truth behind the claim. But this always backfires: Judges more than anyone know that lawyers are advocates. As the saying goes, “When you have the facts on your side, pound on the facts. When you have the law on your side, pound on the law. When you have neither on your side, pound on the table.” Using words like “obviously” and “clearly” is just the written version of pounding on the table.