Maybe give people unlimited upvotes but a budget of downvotes, say 3 per day or 3 per article. This wouldn't prevent multiple accounts colluding to brigade a thread or person, but it might help stop the casual, uncoordinated drive-by downvoting.

Or, make people actually type in why they're downvoting before it counts. Again, wouldn't stop a motivated attacker or coordinated attackers, but could add just the right amount of friction.