I was one of the persons who voted to re-open the posting in question, so let me take a minute to explain my reasons.
Basically, I disagree with the claim that the posting in question is "so opinion-based that it can't be answered objectively". The posting obviously expresses some very strongly held opinions, and I will readily note that not every locution was felicitous. However, as long as one isn't too put off by its tone or, worse, treats the tone as an invitation to engage in a shouting match, a lot can be learned from reflecting on the merits of its claims, bothering to write an answer, and/or reading the other answers that have been posted. The answers that have been posted so far, for the most part, do strongly take issue with one or more of the claims made by the OP, and in a reasoned manner. Taken together, IMO the answers make a lot of useful and interesting points, and "useful" and "interesting" are desirable attributes, in my book.
I will not be drawn into a debate about what the word "objective" may mean. Obviously, the word has a lot of possible meanings, and little is to be gained from claiming that one's favorite meaning of the word is the best possible one. Naturally, I do believe that there are lines that shouldn't be crossed with regard to postings that may fit the scope of this site. Some postings to me clearly lack either objectivity or the potential to elicit objective (or, at least, useful and interesting) answers, and I have no problems with voting to close such postings -- and with resisting calls to re-open them.
I don't know how to address your second question, except to say that I'm confident that I know a non-objective posting when I see one -- and will vote to close it.