There are two discussions here, the academic discussion and the practical discussion.
As a threshold matter, I tend to agree with you on most accounts. And, as I stated, I haven't seen anything specifically that I think rises to the level of this discussion, especially about DeBord. What I was saying is that people tend to believe that what you do here is anonymous and that opinions are somehow protected speech.
As to the general academic discussion: All that is generally required is a communication of information that is false and damages someone. With a public figure (such as a coach or player) the plaintiff might have to prove that the falsehood was communicated with malice (that is that you knew what you said was wrong but said it anyway). Case law and modern interpretation of these issues show that even when someone says 'it is just my opinion', or the statement was made as an opinion, they are not necessarily immune or protected. Further, slander regards the spoken word. Libel is generally anything that is written, including message board posts and even TV broadcasts (as they are generally considered as transcripts and not spoken word).
The practical discussion is that none of that matters. Few on this board can likely afford the defense that would be required against a defamation lawsuit. Simply by suing you, the plaintiff can force you to spend thousands, if not tens of thousands, of dollars. Thus, the pound of flesh has been extracted even if the case never goes to court. The fact that many on here couldn't afford the defense also makes it less likely to spend the money to sue them to begin with, but I wouldn't advise on using that as a shield.
Further, as this basically a permanent public record, I caution anyone against making bigoted or careless statements, especially about drinking or drug use. Like it or not, your current or future employers could intentionally or accidentally stumble across something said here, and that could very well make one as unhireable as many on volnation believes being a bald fat white guy makes a coach.
Just food for thought.