I actually made a reply on a similar topic about having the ability to vote against proposals.
Personally I feel like we should be able to vote up or down a proposal with our stake. At least then we can show any disagreement, but it does however have its own problems with large stake holders holding all the power over proposals but then again it's not much different from now. I think we should trial such a system see how user behavour plays out and go from there.
Not being able to vote down a proposal really doesn't make sense.
When it comes to money and funding we need a way to disagree with other people without having to resort to raising the bar for all proposals by voting the return proposal. Right now we have 3 vote options on proposals, agree, abstain and 'Raise the bar'