Have you ever noticed that some people, when confronted by a new idea, always seem only to find what's wrong with it? Admittedly many, perhaps even most, innovative or novel suggestions have problems, bugs, infelicities, or fatal flaws, but some people seem to delight in finding what's wrong.
Perhaps they're simply trying to kill anything new to make sure they don't get knocked out of their comfort zone. Perhaps they genuinely believe that new ideas should be tested to destruction. Whatever the reason, some people are unremittingly negative.
That kills innovation.
Don't get me wrong, new ideas need to be examined from all sides. I've learned from bitter personal experience that business proposals need to be tested in the fires of relentless criticism and nay-saying. But when it's entirely negative, the target will either give up, or will ignore what might prove to be valuable advice.
Strive to be balanced. Find the good points in any idea. Maybe the proposal you're facing has got fatal flaws, but maybe there's the germ of a world-changing idea or invention that you're missing.
See if you can find it.
Then go one step further and really prove your worth. When you do find negative points, and I'm sure you will, don't just play the "I'm cleverer than you" game. Don't hint, don't tease, don't imply, and don't indulge in one-upmanship. Instead, be constructive!
Some people, many people, even perhaps most people just say, "No, that'll never work, and here's why." Instead of being one of them, find something that will move the idea forward. Don't be content to be a demolisher of ideas. Work hard to be someone that helps to create.
What do you want to be remembered for?
Of course, this only works when people do care what others think. It seems to me that a wiki works superbly when people have respect for each other (which means they're polite, even when they disagree) and consider losing that respect to be of negative value. Edits are then cast in a way to retain respect, and usually that's best done by being constructive, helpful, or positive.
Of course, if someone really doesn't care for the respect of the others, they can and will do what they want. If they don't care what they will be remembered for, if they don't care how others perceive them, or if they don't care what others think, their behaviour will not be guided by approval, disapproval, or other feedback.
This is a plea -- don't be like that, do as I say.