One of the things I've noticed is that us on the other side of the world have trouble generating the same kind of karma that other's can generate which is really a function of being too late to discussions.
For example, east coast USA is often around 14 hours behind my timezone, so I typically get to conversations about 6-8 hours after they've started and when people are usually heading to bed.
Generally I find my comments down the bottom of threads with 1 or 2 karma.
However, if I post in the middle of the night (usually as I'm heading to bed) or really early in the morning (essentially as I get up) I am in the peak hours of HN, my comments on average do very well for Karma, simply because more people are available to read them and as such vote them up or down accordingly.
Just a thought - you might want to have thresholds based on a person's "prime time" instead of a predefined average.