Upvotes are easy on both Main and Meta sites. You like something, you vote it up.
Downvotes are trickier.
On Main down-voting a question means that the question is a poor one. Poor meaning badly written or really annoying for some reason (e.g. give-me-da-codez).
Downvoting an answer usually means it's plain wrong.
On Meta, however, many questions revolve around matters of taste (e.g. the HW question). When voting on answers to such questions, I want to be able to push down the suggestions that I disagree with and promote the ones that I think are correct. But I don't want to hurt people's reputation by doing so.
Downvoting on Meta isn't really saying "your answer is stupid and wrong"; it's more like "I prefer we do it the other way". The whole site is basically meant to discuss matters of social preference, etiquette, do's and dont's, etc.
In Main most people wouldn't downvote an answer just because some other answer is better. People generally accept that there may be different solutions to a problem and feel comfortable in letting upvotes alone decide on the most popular one.
In Meta, on the other hand, there might be groups that really care about some issue and would like to see it solved in a particular way. And, in case meta replaces UV, voting on answers would really be voting on preference more than correctness.
Should downvote semantics on Meta be different than on Main?
Perhaps the system should not deduct reputation points if your answer gets downvoted, thereby allowing/encouraging more free downvoting.