As someone who has not seen said 4th season of Sherlock yet I can, however, agree that I have seen a great deal of opinionated comments about the allegedly bad quality of that season on new questions asked about it.
I'm a little at a loss of what exactly you expect from the answers here, though. As you realize yourself, it's not our pupose to discuss the truth behind those statements. Neither do I think it's the first time this happens for a work to garner a lot of criticism in the site's comments (think of the Star Wars prequels for example).
That being said, it's admittedly not that normal for a TV-show to gather such an amount of negative comments. And it is certainly not constructive or encouraged in any way. While comments might be a little less strictly policed than questions and answers, denigrating works for the sole purpose of complaining doesn't really contribute anything useful to the site. And as much as the site does sometimes encourage a more "subjective" approach than other SE sites, stuff like that does not fall under what we call "good subjective". To quote the help center:
Some subjective questions are allowed, but “subjective” does not mean “anything goes”. All subjective questions are expected to be constructive.
There is certainly a difference between:
I don't think there is much of a conclusive answer to this other than mere inconsistency of the story in this regard.
Yeah, that was just one of those things that were completely bollocks in this episode. I'd say the writers smoked the wrong stuff.
Feel free to flag the latter kind of comment as not constructive. For anything in between those two examples, it's upto to case-by-case judgment. But if you deem any kind of comment along those lines probelmatic, feel free to flag it and the moderators will try to assess its value.
And of course we try to avoid assessments of that kind in questions and answers even more. So if you see unnecessary opinionated judgment there, feel free to edit it out.