If I post about how awesome X is, and then a bunch of people join in, also excited about how awesome X is, and we're all dancing in a big circle, waving our arms in the air and shouting "Woohoo for X!" then please don't be the person who stomps in and says "X is rubbish."
Because you're taking something that people are happily enjoying, for their own personal reasons, and mashing it down with a big grumpy hammer.
It doesn't make you cool. It doesn't make people respect your opinions more. It doesn't change anyone's mind (if I actively enjoy X then I'm not going to suddenly cry "What was I thinking? X actually made me feel like someone stole my puppy!"). All it does is give you an outlet for your venting, while reducing the happiness of the people who were, until that point, squeeing at each other.
If you actively hate X then the place to vent about it is on _your_ journal.
This isn't to knock positive contributions. If you want to tell me that if I like X then I might like Y even more, that's great! Deflect my enthusiasm with your enthusiasm, and I'll feel enthusiastic about listening to you!
And it's not to say that legitimate criticisms aren't ok - if X is made from grinding up kittens into paste, and you think there's a chance we don't know about that, then by all means ask if we know.
Discussion is fine. Legitimately asking why I like X, and what I think is good/bad about it, also fine.
But please don't leap into a group of happy people and tell them that they're wrong to like something, because frankly, it's just rude.
(Reviews are a different matter - posting a review which talks about positive and negative aspects of something is asking for discussion of the subject matter. I'm talking specifically about exciting, celebratory posts.)