Note that downvoting a post costs 10 karma from your downvote cap of 4x current karma.

Does it mean 10/4 actual karma (10 points of downvote cap), or 10*4 points of downvote cap (10 points of actual karma)? (I expect the former.)

The number of remaining downvote points should be visible, otherwise at some point people run into a brick wall that should be avoided by calibrating the rate of downvoting rather than by being unable to make high-certainty downvotes. Even better, allow unrestricted downvoting, but recalculate the efficacy of downvotes based on voter's karma and number of downvotes.