Yeah Im pretty sure that critics would be out of business if an opinion constituted Libel.  It's whether or not the opinion is presented as fact, not whether or not the opinion is based on fact. 
Again with the legal expert thing..

wikipedia ftw

""Another important aspect of defamation is the difference between fact and opinion. Statements made as "facts" are frequently actionable defamation. Statements of opinion or pure opinion are not actionable. In order to win damages in a libel case, the plaintiff must first show that the statements were "statements of fact or mixed statements of opinion and fact" and second that these statements were false. Conversely, a typical defense to defamation is that the statements are opinion. One of the major tests to distinguish whether a statement is fact or opinion is whether the statement can be proved true or false in a court of law. If the statement can be proved true or false, then, on that basis, the case will be heard by a jury to determine whether it is true or false. If the statement cannot be proved true or false, the court may dismiss the libel case without it ever going to a jury to find facts in the case."

Rendo should have a permanant link to the wiki article as often as that is brought up here.

But for example:
If someone blatantly lies about a member in a forum.  It is obviously a provable lie.  Then the member takes that issue to Moderators and they do nothing, not even respond to the member.  Then rendo would be in liable.  

I think I have dejavu...

I am:  aka Velocity3d