I have to agree that when someone disagrees, they should have a decent reason posted as to why. Especially when the OP is talking about ideas or asking for criticism. Posting offensive, stupid shit against the OP's topic/idea only dishearten's others from posting if they realize that they could be crucified on the forums for having a bad idea.
Just don't respond if you don't have anything constructive to say. Of course people can disagree, but don't be a dick about it.
Now, one thing to take into consideration, is that the average age of Minecraft players has lowered to around to the teens...like 12-16 yr olds. That, by default, will warrant non-constructive posts. My first minecraft server that I joined way back in Nov. 2010 was vastly populated by 18+ just because Minecraft alpha/beta 1.0 was unknown to the mainstream crowd. I ended up being a staff member on that server (not anymore) and had to quit b/c I became a babysitter. Many of the old fart veteran players left b/c of the kids playing. My head imploded by the amount of children applying. Now...well now parents are finding out that there is this cute lego-like game that they can let their kid play. Those kids eventually find themselves on popular servers griefing left and right and on the forums contributing jack shit.
I also think that this rule shouldn't be applied to all forums. Maybe the forums like Off-Topic, Other Games, and the less serious forums should be more lax in a sense. I think the general nature of those forums kind of warrant non-seriousness lol.