I've just read that downvoting is used to ban poor askers. However, there are many, many members who will never dare downvote anything because of that little 1 rep. (Besides, I think downvoting is not taken into account in the 200 daily reputation limit, but I'm not sure.) This has, as a side effect, that only really, really bad questions get downvoted.
I understand that downvotes remove rep from the people who receive it, so it must not be abused. But maybe users could have, like,
(reputation / 1000) free downvotes every day?
Currently, the incentive for downvoting is a strong sense of duty. The incentive for not downvoting is OHEMGEE LOOK AT MY REP. And often, the latter is stronger.