Ah, the dreaded "I'm entitled to my opinion" vs. "If you don't like it don't watch it" debate
I think as long as it's reasonable people should feel free to say whatever they like. It would be nice if people would notice when they themselves start treading into the extremes of the spectrum. That would probably nip most of this problem in the bud. Too bad it generally takes someone else to point it out for them, which tends to cause the conditions necessary for a flamewar.
I think one thing that ticks a lot of people off is repeating the same kinds of things over and over every time a new episode comes out in a series. Doing stuff like that just begs the opposite extreme to get involved in a less than constructive manner. It's generally more apparent when it's repeated bashing of a show rather than praising it, but both can be pretty disruptive to a thread.