My personal view is that -- means "this should not have been posted". It's a stronger statement than ++, in spite of its appearing to be exactly the opposite (and the fact that it is mathematically the opposite). 5 --s might get your node reaped. There's no analogue for that at 5 ++s.

I agree with those who point out that a personal reply explaining the downvote might lead to retaliation. I would like to see an anonymous downvote explanation box, though, so that those who downvoted could explain why. If they don't have a reason better than "your node sucks", they're probably unjustified in downvoting. At least you'd know not to fret that you did something wrong. All the downvote explanations would be grouped together as if they're one reply, since each is only one line, and they're anonymous.

Then, in this World of What I'd Like To See, maybe the downvotes themselves could be downvoted (at no cost to your personal vote hoard) by those who considered them spurious. The especially spurious ones would be removed (and the point restored to the original node). But that's a whole new can of worms, and extra programming effort for the site, besides.