In other words - make votes precious.
That way people will think more about how to 'spend' their precious votes.
A similar thing works in poker. If you empty out your change jar and give everyone a fixed amount to start, and at the end of the game it all goes back in the jar, people play in a certain way. If you play for actual money, even just change, the gameplay does often change for the better, because their chips now have value.
At the moment we all have an infinite amount of votes to spend, so we can casually upvote anything we find briefly interesting - because our votes have no value to us.
By limiting the number available per day, we are forced to spend our votes more wisely.
Alternatively, making upvote decrement our karma will also add perceived value to the action of voting. However I think HN users care less about their karma scores so I think this approach wouldn't work as well as limiting users to N votes per 24 hours.
N can be fixed at, say 10, or increase with karma so 'better' users get more votes and thus more influence.