I'm of the opinion that you can't adequately quantify somebody's reputation on a forum with upvotes and downvotes. While some more adult-oriented communities (such as a camera forum I frequent) are able to have a system like this without it being abused, I've never seen it result in really meaningful ratings that are indicative of those peoples' overall reputations.
1: Restrict reputation use to accounts older then one year.
Some of our members who behave most immaturely have been around the longest. In fact, most of the people who caused problems and made a joke out of the rep system the first time we tried it were long-time members.
2: Require a comment to be made along with a rep vote. (Other sites have used this and it cuts down on needless downvoting.)
I won't lie, I have a low opinion of the maturity of this community as a whole, which is proven time and time again to be accurate. So in designing new features, I think in terms of worst case scenarios: will the lowest common denominator side of the community abuse this new feature? With this suggestion, it's hard for me to see how it wouldn't result in people making frivolous posts just to get their vote in. We already have a major problem with people posting just to (needlessly and harshly) criticize somebody else's ideas, I don't want to add new features with precautions designed in such a way as to encourage that.