Copied shamelessly of Anna Lear answer which is also my standing (through previously didn't express it well):
If you really care about enforcing integrity in voting, you should also be asking for explanation of upvotes. Some people upvote with good reason, some out of sympathy if they see a downvote, some because they didn't even read your post but think it must be good if it already has X upvotes, etc.
In the end, the anonymity of the voting system is what makes these sites useful. Always having to explain downvotes would destroy their anonymity and expose downvoters to potentially harsh criticism from those who disagree with the downvote. Not to mention that knowing that you have to explain it (even if an explanation already exists or is obvious) would further discourage people from downvoting.
The system right now is simple. Think a post is useful? Upvote. Think a post is unclear/not useful? Downvote. It doesn't need to be more than that.
Most of the emphasis was mine ;)
So, what's the problem here? First, people here doesn't vote enough... not up or down, if you ask them for a second click they will vote less. Second, people here is too nice to downvote. Third, people is not honest enough to downvote bad, crappy, <insert your subjective reason here> posts. Fourth, they are already several guides about how to write good and awesome answers and questions. If I have to explain each time why I downvote or upvote something obvious, we are doing it wrong and people will steer away Ask Ubuntu (or Stack Exchange for what matters).
The reason to upvoting or downvoting posts are in a nice tooltip over the button: This question doesn't show research effort; it's unclear or not useful (for questions) and This answer is not useful (for answers) that also could mean "you are wrong".
If anyone wants a compiled list of reasons why people downvote questions of answers, please check Jon Skeet's questions: