Not that I do this, but isn't there a limit to how much karma an account can add to or remove from other posts? One based on how much karma that account has gotten? So if someone is using two accounts to do multiple up/down votes to the same comment, isn't this already limited by the fact that they have to earn karma in the second account first?

I would be much more interested in a feature that discounts (but not entirely) the effect of votes when the voter is participating in the thread whose comment they voted on. Such voting is suggestive that the user is doing an "I disagree for the wrong reasons/ I don't like you" vote, which is disinformative.

This is similar to Slashdot's system of not allowing you to moderate and post in the same discussion. (I think that goes too far in this direction, but has the right general idea.) It's also a good policy to adhere to on an individual level, as it prevents you from voting when you're biased.