[wvenable]

I've thought this as well but you probably only need 3 buttons: agree, disagree, and inappropriate.

Basically pressing either agree or disagree is also saying that it contributes to the discussion. Clicking inappropriate is the same as downvoting as it is now. Finding the right terminology might be difficult.

I'd love to see this as an experiment to see how it changes the discussions here.

[DanBC]

Isn't this what HN has? Up, down, and flag?

[wvenable]

No because "down" doesn't mean "disagree" -- down votes cause comments to sort lower and negative amounts will gray out the comment. "Flag" kills the comment if deemed by moderators as inappropriate.

In this proposal, "disagree" and "agree" are both the same as "up".