Downvotes are problematic, because even the best people can't always be trusted to use them correctly. A downvote should be reserved for something offtopic, evil, mean, etc., but too often a downvote simply means "I disagree".
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I agree the potential for damage is much higher with downvotes, but there's an asymmetry there.
The trick is to make downvotes cost a little bit of karma, IMO. And while I'm on the topic, upvotes shouldn't be as free as they are, either.
Having said that, the 2-1 downvote scheme you proposed does put some of us within spitting distance of taking nickb down to zero, so I support it wholeheartedly.
It's like people used to say about micropayments; all these little knobs exact a mental cost; I have to spend 5 cycles deciding what to do --- mod up? mod down? what are other people doing? do I need to care?
Seems to me like Hacker News is doing the Simplest Thing That Could Possibly work. A good thing.
Wouldn't it be simpler to automate the whole thing, and let the algorithm do the work?