Hot and bothered.. not in the slightest. You ARE most definitely entitled to your opinion...
I *think* the negative reactions you, or anyone gets when they post after a 1st preview is because at this stage of the show's life cycle, a complete damning or slamming of a production often doesn't take into consideration (holistically) that there is time and the potential to FIX the show. I just think it's a human dynamic to "approve" of positive feedback and the things that work for people right out of the gate, rather than a swarm of negative. And to be fair, you said the production had no redeeming qualities. People are going to jump on that, I think, because it's almost impossible to believe any show, no matter how bad, has no redeeming qualities.
Additionally, people react to the fact that when they post a contrary opinion, they are not given the same weight as someone who, like you, thinks its crap.
Everyone has different tastes - and just because you, or anyone things something is crap, or utterly fantastic, doesn't mean that's the definitive opinion. That's what many people react to as well. Sometimes it's not what one says, but how one says it.
I happen to love Chocolate. I have a friend who thinks it's absolutely disgusting. To me I find that unfathomanable. I mean - there are people who don't care for it or choose other things to eat - but disgusting? I think that's crazy talk :) And I would gather that the majority of people would disagree with my friend vehemently. But again, every has different experiences and viewpoints. Who would want to live in a world where everyone thought exactly the way you did? Boring....
"A little nonsense now and then is relished by the wisest men" - Willy Wonka