Out-law, bloggers in the USA are better off than in the UK in terms of the risk of being sued or gagged, after a case where an Ilena Rosenthal was sued by a Dr Barrett in the US after she posted a supposedly libellous email about two doctors to a newsgroup (but without changing it).
If you publish someone else's libellous statements, even after you've been warned that they're false and libellous, because of Section 230 of the Communications Decency Act the US courts won't entertain a lawsuit against you - the person complaining that they've been defamed has to sue whoever originally made the libellous statement in the first place, not other people who've simply allowed that comment to be published e.g. in a blog, website, discussion board or - in this case - passively passed it on to a newsgroup.
Contrast the position in the UK. The Out-law article says "In the UK, service providers have a duty to remove or block access to defamatory material that they host once they are made aware of it. They can be sued if they fail to do so expeditiously... The same approach is taken across Europe; but the US considers this to be an impossible burden on service providers and anathema to free speech."
So bloggers and others in the UK who passively pass on a potentially libellous statement can be shut up or gagged merely by someone complaining (rightly or wrongly) to the ISP or host that the statement concerned was libellous; whereas in the USA they can't be silenced by the courts (unless they're the original source of the defamatory comment, in which case they can be sued). This does as Out-law observes make it harder for someone who's a true victim of a false and defamatory statement to stop its publication by others by suing in the US courts, though at least they can still sue the person who originally made the statement.
But it's not a blanket licence to US bloggers to excuse themselves by saying "It weren't me who first said it, guv" (oh awright, I know you Americans don't say "guv"!). The US court did point out that "At some point, active involvement in the creation of a defamatory Internet posting would expose a defendant to liability as an original source. Because Rosenthal made no changes in the article she republished on the newsgroups, we need not consider when that line is crossed." So I guess if you not only repeated someone else's criticism or other defamatory comment but added "Too right!", and more so if you embellished the statement, then you're still at risk of being sued in the US. It still pays to be cautious about what you say or repeat in your blog, I think.