This style of site is a good one, but you might take a few pointers from some other, similar sites. For example: http://brainstorm.ubuntu.com/ . They have three different options to vote: Disagree, Agree, and Abstain.
Beyond that, a mistaken mis-click is easily remedied, by clicking on another option. Sadly, I've already mis-clicked a couple ideas in this forum, so it would be handy to have.