That doesn't seem like a very nice assumption, that new users will have bad ideas, and if old users lose karma every time they vote up a new user, how can you ever know if the new user genuinely has nothing to offer, or if old users are just wary of decreasing their score, and therefore, influence? (i.e., I have lots of karma, and I know if I upvote this comment that I like, it will increase their score quite a bit - I have influence now - but it will also lower my karma, and I'll have less influence over the next comment I see which I may like more...)

This is what happens in chess: highly-ranked players just simply won't play new or lower-ranked players at all. It's not a bad thing there, but then again, it is a competition. A social site isn't, though.