I've recently begun downvoting comments that are at -2 rating regardless of my feelings about them. I instituted this policy after observing that a significant number of comments reach -2 but fail to be pushed over to -3, which I'm attributing to the threshold being too much of a psychological barrier for many people to penetrate; they don't want to be 'the one to push the button'. This is an extension of my RL policy of taking 'the last' of something laid out for communal use (coffee, donuts, cups, etc.). If the comment thread really needs to be visible, I expect others will vote it back up.
Edit: It's likely that most of the negative response to this comment centers around the phrase "regardless of my feelings about them." I now consider this to be too strong a statement with regards to my implemented actions. I do read the comment to make sure I don't consider it any good, and doubt I would perversely vote something down even if I wanted to see more of it.
I am tempted to downvote this comment from -2 just for the irony, but I don't prefer to see fewer comments like this, so I won't.
Besides, the default cutoff is at -4, not -3.
The title says it all.