Speaking for myself, I love constructive criticism, especially when it's in an area I don't see or understand.
However, the posters who bother me, and I suspect others as well, are known to have an agenda. They're not just discussing legitimate concerns, rather, they're trying to justify their opinion that said coach isn't the right coach or said player shouldn't be playing, etc. Sometimes they've already said it, sometimes they're laying the groundwork leading to it. And they do it constantly, about every coach.
Of course, some are just constantly negative about anything and everything.