Thanks for the post Tim. I think the core of the problem lies in the fact that there is an opportunity cost (voting power) to downvote chronically abusive users. You lose the opportunity to reward others - or yourself - when you downvote spammers, etc.
I don't have an answer to solving it, other than potentially reducing or somehow eliminating the costs to downvote others - it's just an observation.
Maybe a separate "downvoting pool?" But maybe that's just over complicating the problem.
There are also no other guidelines to what is and what is not acceptable (this thread comes to mind) and I don't think people can be expected not to upvote themselves if given the opportunity. (This is less of a problem for newer users since the upvotes aren't worth as much anyway.)