To prove libel, which is the same thing as written defamation, the plaintiff has to prove that the blogger published a false statement of fact about the plaintiff that harmed the plaintiff's reputation. Let's break that down.
"Published" means that at least one other person may have read the blog. That's right, just one.
A "false statement of fact" is a statement about the plaintiff that is not true. Truth is the best defense against libel. An opinion is also a defense against libel. But, depending on the context, the difference between an opinion and a statement of fact can be remarkably gray. Context is a big deal in determining defamation.
One thing to watch out for: simply inserting the words "in my opinion" in front of a statement of fact doesn't magically make it an opinion.
Satire and hyperbole can also be defenses against libel, but again, very gray.
Then there's the matter of "harming the plaintiff's reputation." It's one thing to say that a false statement harmed your reputation, but if you can't demonstrate damages, the suit may be effectively worthless. Damages would include, for example, losing X customers that represent Y income, suffering emotional distress and so on. Also, if your damages are minimal, you may have a hard time finding a lawyer to take the case. They're a greedy lot. (That's an opinion, not a statement of fact.)
If the plaintiff is your average, everyday, run-of-the-mill person or company, then negligence is sufficient to prove libel. That means that a reasonable person would not have published the defamatory statement. If the plaintiff is a "public figure," however, then the plaintiff must prove actual malice--a higher burden of proof. That means that the blogger knew that the statement wasn't true or didn't care.
Then there's the question of who's responsible for comments on a blog. Whoever publishes the Web site is responsible for content on the site. That includes comments. However, many bloggers have independent agreements to indemnify the site that publishes their blog. That may or may not include comments.
Plaintiffs can certainly sue everybody in the chain and see what sticks, though they will likely go after those with the deepest pockets. You can avoid the entire question by turning comments off.
