[techiferous]

"I don't think upvoting-on-agreement problem can be fixed technically."

I think there is a technical fix: two orthogonal upvotes. One upvote/downvote pair for agreement, another for whether the post/comment added value. It's a pretty simple fix and I think it would work, but it could have other unwanted side-effects.

[stcredzero]

People would just hit both down arrows.

[techiferous]

Very good point.

This could still perhaps be solved technically, although it would take a lot of effort and is probably not worth it.

You can detect cheaters. People who give up/downvotes on the "value add" dimension that are not well correlated with the community's opinion of value add would be detected as cheaters, or at least detected as users who do not share the same values as the community.

Much harder to do, but possible, is to detect cheating in agreement by analyzing trends. Users who agreed with story A, B, and C may have a 90% chance of agreeing with story D. A user who bucks these trends enough can be detected as cheating or randomly voting.

[DanBC]

A few people would, but many others would be able to say "This is a quality comment, but stupid idea".

[stcredzero]

The site could display the "down-correlation" number, which will be high if someone consistently hits both down arrows at once.