Whether it's a blog or a network or a community tool addon thingy, what you need most is to let people know that their contributions are both need and likely not redundant to others. Polls can get the ball rolling, but they are a gamble. Nothing is sadder than a three-day-old poll with two votes. Nothing is stopping you from boosting those numbers behind the scenes, though.

Don't expect people to chime in in a reaction to a specifically popular or unpopular opinion. "Me too" and "Screw you", despite their popularity in popular spots, are no fun to write and won't be written, even if strongly felt, in an empty room. Expressing a unique, rare, and non-antagonistic opinion, and then asking for others, is more likely to get reactions than "I like music, do you like music?" or "Taxes suck, am I right?"

Also, invite pride. If people have profiles or pages, let them create something that they can point others too. Allow for long, weird contributions.