Well, lets flesh out the full hypo:
In the real world, a person can put a sign up in their front lawn saying "McDonalds is the worst tasting food ever" and no you can't touch that because it is opinion. It doesn't matter if it is anonymous or not.
On the internet, a person can do the same, with the same results.
But in both the real world, and the internet, if you write "This McDonalds puts human flesh in its burgers", you can be sued for defamation.
The court/jury will look at factors including whether or not it is parody and of course, whether or not it is true.
I don't see why posting on the internet (as apposed to placing a sign in your yard) should give you any sort of immunity from prosecution.
There is a legal PROCESS. If you didn't do anything wrong, you will be acquitted.
The only difference between print and the internet is that in theory you can print 1000 flyers defaming McDonalds, not put your name on it, and no one will ever know who spread them around central park. You can "get away" with it.
But similarly, you can go to a public library, make an internet post defaming McDonalds, and "get away" with it.
The point is that I see no reason why the internet should be different than print, anonymous or not.
That said, the court SHOULD AND MUST respect the difference between opinion and fact.
I can say, "The resturant should have been cleaner" and that is an opinion.. and should be protected on both the internet and print. (Unless maybe they can prove it was a Level 3 clean room, I kid, I kid)