Well, calling Obama a murder COULD possibly get you in trouble depending how you went about it, but it would require a far higher burden for the government to manuever over to be able to do anything about you. This is because he's a public figure, and as seemingly known commodoties the proof of harm from claims against them is much higher.
If I say Christian Bale is a murderer, most of those that hear it would have reason to be immedietely HIGHLY skeptical and not believe it. He's a famous actor, he's covered by magazines and shows and papers all over. He's well within the public eye. If he killed someone it would be massive news that people would be highly likely to know about, so there's little reason for my random statement about him to be taken serious by almost anyone and thus would have little to no likely harmful effect on him.
If I say Bill, my next door neighbor, is a murderer then that's a different situation. People have less reasons to be immedietely highly skeptical of me and not believe it. He's not famous, he's not well known, there isn't any great amount of coverage on him and his life. There's no reason for a random person I talk to at work to know whether or not "Bill" is a murderer or not. There is a far greater chance of damage being caused to him due to that defamation because there is not a preconcieved general understanding regarding him in the greater public at large.
To go further on that point as well, context of course matters greatly. If I called my neighbor "Bill" a murder in a statement saying "I can't beleive my neighbor eats so much steak all the time, the man is a murderer with all the cows that have been slaughtered to feed him", then there's likely nothing to come of it even though he's not a public figure. In the context, it's clear I'm stating an opinion regarding the issue of eating animal meat and not making a statement suggesting a literal accusation of murder, the legal term.
Similarly, most of the time when people are talking about "Obama" or "Bush" being a "murderer", it's actually statement of opinion and not a claim of concrete legal fact...typically resolving around things like military action or laws they've passed. Combined with the higher standard for public figures and the ability to show LEGITIMATE HARM being done is extremely problematic.
Which DOES go back to the notion that our rights are meant to be generally protecetd from infringement by the government save for instances where the infringment is necessary to prevent the realistic harm of others.