I definitely use the down vote button a lot to expressive negative affect, which is strongly influenced by ‘disagree’. Having separate buttons could be pretty awesome.
You could give a little (3x3? 5x5?) voting grid: usefulness is one dimension, agreement is another. Users have the option of hiding one of the dimensions, and maybe this is the default.
I definitely use the down vote button a lot to expressive negative affect, which is strongly influenced by ‘disagree’. Having separate buttons could be pretty awesome.
It would also be terrible UI to have two ‘up’ and two ‘down’ buttons with different meanings.
The nearest thing that would be feasible would be to have, as does LessWrong, a polling feature.
You could give a little (3x3? 5x5?) voting grid: usefulness is one dimension, agreement is another. Users have the option of hiding one of the dimensions, and maybe this is the default.