You make a decent point but there's a good reason for the downvotes. Downvoting before reading is not in the spirit of this community (though sometimes, in rare cases it may be okay). You should not be here to collect karma like some sort of street cred so you can play god with the comments. There's a lot of karma policing going on lately where people think that button is there so they can push down stuff they don't agree with. That isn't the point. Up and down votes should be to everyone's benefits and unfortunately there are ton of people lAtely using them on some power trip. Don't be one of the "Karma Police". Use the buttons judiciously.

Right, but why can't I downvote overall submissions? I understand there is a karma threshold for downvoting comments, but I'm unaware of any mechanism for submissions.

Well, think of it this way: if you don't have anything nice to say then don't say anything at all. The cream always rises. I think it's great that all submissions have an equal chance of getting noticed. If enough like it, it goes up while inevitably surpassing the crap that languishes at the bottom. Most unpopular articles don't need any help getting ignored.