Since you mention Slashdot, here's a little side effect of one of their moderation systems. At one point, they decided that "funny" shouldn't give posters karma. However, given the per-post karma cap of 5, this can prevent karma-giving moderation while encouraging karma-deleting moderation by people who think the comment overrated, potentially costing the poster tons of karma. As such, moderators unwilling to penalize posters for making jokes largely abandoned the "funny" tag in favor of alternatives.

I suspect that if an agree/disagree moderation option were added, it would likely suffer from a similar problem. Eg if we treated that tag reasonably and used it to try to separate karma gains/losses from personal agreement/disagreement, people would be tempted to rate a post they especially like as disagree/love/awe.

A more interesting idea, I think, would be to run correlations between your votes and various other bits, such as keywords, author, and other voters, to increase the visibility of posts you like and decrease the visibility of posts you don't like. This would encourage honest and frequent voting, and diversity. Conversely, it would cause people to overestimate the community's agreement with them (more than they would by default).