And if so (uninformed legal commentary here), is it illegal to express negative opinions in writing? "My landlord doesn't care that my apartment is moldy" seems pretty tame and commonplace.
The company would here argue that her tweet asserts that company policy deems it acceptable to expose tenants to serious health risks. It would say this is false and highly injurious to its reputation in the community.
Had this firestorm not arisen, the company might easily win through intimidation in such a lawsuit by coercing a settlement. A court could let the case get to a full trial, meaning that it would (if nothing else) take many thousands of dollars to defend it.
For this reason, it is not wise to use Twitter as a forum to vent in this way.
That said as a general matter, in this case, the company's patently idiotic response did far more to hurt its own reputation than the original statement possibly could have done. Unless it has lost its mind, it will drop the case pronto.
If it is a mere opinion, it would not constitute libel. For something to be libelous, it must be an assertion of fact - it must also be false and tend to injure the reputation of the party within the community.
Isn't every opinion an assertion of fact? If I say "American cars suck" or "Apple mistreats its developer community", I'm expressing what I believe to be the truth, not just my opinion.
What's the distinction between opinions and assertions of fact?
You make an excellent point, though, in picking up on this issue. No less an authority than the U.S. Supreme Court (in the 1990 Milkovich case) expressly rejected what it called "the creation of an artificial dichotomy" between fact and opinion and held that statements couched as opinions can be libelous if they imply false and defamatory facts.
So, if you say, "In my opinion, politician x is a drug dealer," your statement could be found to be defamatory.
For a good discussion on this, see http://www.citmedialaw.org/legal-guide/opinion-and-fair-comm....