Low-quality posts are a constant annoyance (which is why it constantly comes up in Meta and chat.)
The problem that I see is two-fold:
It costs you rep. to down-vote a low-quality post, which (like it or not) is a deterrent
It costs the person receiving the down-vote relatively little.
To illustrate the problem with an extreme example, allow me to paint a hypothetical picture. Suppose someone asks a question about the accuracy of a claim in Scripture. Suppose then that a non-Christian posts an answer to the effect of "Scripture is a fairy tale" but the answer is not blatant enough to warrant deletion. Now suppose a couple of sympathizers up-vote the answer. The person answering now has +20 reputation on the site. If 9 Christian experts, moderators, etc. were to down-vote the answer, the person answering still walks away with an increase in reputation, and the 9 down-voters are penalized!
My Proposal: A down-vote should cost the person posting at least as many points as an up-vote would earn them. That way, if a person is consistently posting garbage, the person will lose reputation and privileges over time, instead of gaining reputation and privileges over time as with the current system.